Sometimes it’s not a team’s winning or losing that surprises, but rather the way they’re winning and losing. For instance, that the Nets got off to a rocky start is not particularly surprising, especially given the absence of Vince Carter for several games. But had the Nets been losing games 135–125, that would have caught our attention.

So far in this young season, it turns out that several of the league’s contenders are doing that very thing. Offensive juggernauts have suddenly discovered religion at the defensive end, while some of the league’s more buttoned-down clubs have unexpectedly let their hair down.

Take the defending champions, for instance. The Spurs normally win with suffocating defense, having ranked third in the league in Defensive Efficiency (my measure of a team’s points allowed per 100 possessions) after leading the league four of the previous five seasons. They uncharacteristically find themselves in seventh entering Monday’s games, yet San Antonio is winning as much as ever thanks to a newfound offensive spark — with Manu Ginobili playing MVP-caliber basketball and Tony Parker not far behind, the Spurs are second in the NBA in Offensive Efficiency.

Similarly, the league’s top five in Defensive Efficiency is a laundry list of eyebrow-raising names. You have the Utah Jazz at fifth — a fine basketball team, but one that ranked 19th in this category a year ago and won only because of an incredibly efficient offensive attack. Then there’s Byron Scott’s Hornets at fourth, a solid if unspectacular defensive team a year ago that lost its best wing defender to free agency and returned an immobile jump shooter to the lineup.

At no. 3 in Defensive Efficiency is an even bigger shocker — Phoenix. The Suns were supposed to struggle at the defensive end since the trade of Kurt Thomas left them without a quality interior defender. Instead, they’ve used the improved D as the foundation of an eight-game winning streak. The second-best defensive squad? Would you believe … Denver? Yes, they have the reigning Defensive Player of the Year in Marcus Camby, but they also have two of the game’s most notorious cherry-pickers in Carmelo Anthony and Allen Iverson, and not a quality wing defender to be found. Clearly, there’s something in the water this year.

But the biggest surprise of all is the no. 1 ranked defensive team: the Boston Celtics. The key to their scorching-hot 11–1 start has been a shockingly good defensive effort. Boston allows only 92.9 points per 100 possessions, more than three points better than the second-place Nuggets. That would make them a fantastic team even with an average offense, but of course Boston’s offense is well above average thanks to the star trio of Paul Pierce, Kevin Garnett, and Ray Allen, ranking fifth in Offensive Efficiency.

Needless to say, Boston is crushing opponents on a nightly basis — with such dominance at both ends, why wouldn’t they? Nobody doubted they could put together a potent offensive squad, but the defensive results are downright shocking. And they bear closer examination, because if the Celtics keep defending this way, it’s hard to imagine them not winning the championship.

One way the Celtics excel is by limiting their opponents’ shots. Boston is the fourth-best defensive rebounding team in the league, grabbing 76.5% of opponent misses. With a board beast like Garnett joining another strong rebounder in center Kendrick Perkins, this is perhaps not a huge surprise. A bigger surprise is the turnovers they’re forcing — Boston opponents cough it up on 17.9% of opponent possessions — again, the fourth-best number in basketball.

As a result, Boston gives up fewer “shots” (defined as field-goal attempts or trips to the foul line) per opponent possession than any team in basketball. This alone saves them more than two points a game compared to the average team, a huge total over the course of the season. By the way, the Nets are a close second in this category, thanks to their league-best Defensive Rebound Rate.

So what’s the difference between the Celtics and the Nets? Basically, Boston’s opponents miss a lot more shots than New Jersey’s. The Celtics are no. 1 with a bullet in field-goal percentage defense, with opponents converting only 40.9% from the floor. This is miles ahead of the next-best team (Los Angeles Lakers, 43.5%), accounting for the other big reason Boston is so far ahead of the pack in Defensive Efficiency.

It’s also shocking, much more so than the rebound and turnover numbers. Boston doesn’t strike anybody as a shutdown defensive club. While Garnett is a good defender, the Celtics don’t have much other quality size inside, especially with Perkins in frequent foul trouble. On the wings, reserve James Posey is the closest thing they have to a defensive stopper, and his minutes are limited because Allen and Pierce are on the court so much. Allen is a notoriously poor defender, and Boston’s bench brigade is generally lightly regarded at the defensive end too, especially reserve guard Eddie House.

But somehow, the whole vastly exceeds the sum of its parts so far. Some ascribe it to the teachings of new defensive assistant Tom Thibodeau — who was in Houston helping the Rockets rank no.2 in Defensive Efficiency last season. Some say it’s the esprit de corps resulting from the excitement surrounding the rebuilt Celtics, or the impact of the ultra-intense Garnett on his teammates.

Here’s a possibility we need to at least consider — that some of it might be luck. Field-goal percentage is a notoriously fluky statistic, especially when looking at small samples. Boston’s rebound and turnover data indicate that it’s unquestionably a quality defense, but if they keep suffocating teams like this they’ll go down as one of the best defensive teams of all time.

Additionally, opponent 3-point percentage is even more subject to random short-term fluctuations, and Boston’s percentage allowed there is also exceedingly low (31.7%). Finally, Boston is fouling far more than the league average and giving up an above average number of 3-point attempts, two factors not normally associated with dominating defenses.

As a result, we have to treat Boston’s early-season defensive dominance with some skepticism. At the same time, understand that this doesn’t mean they’re suddenly going to start giving up 120 points a night. If Boston merely ends up as the fourth- or fifth-best defensive team instead of the best, it’s still likely to dominate the East and have a great shot at winning the title. Even if the insanely low opponent field-goal percentage doesn’t hold up all season, one presumes they can at least accomplish that.

jhollinger@nysun.com