What has Seattle been putting in its lattes this season? That’s the question for those trying to piece together the Sonics’ stunning turnaround. Friday’s win over the defending champion Pistons served to cement the now-obvious fact that Seattle isn’t going away. At 41-16, they’ve all but clinched the Northwest Division with the NBA’s fourth best record.

It’s hard to fathom how this team became such a juggernaut. Many basketball observers pegged the Sonics for the Western Conference cellar in 2004-05; they were coming off a 37-45 season and faced significant uncertainty about the future of their players and coaches. Seattle faced such a variety of obstacles, in fact, that few teams would have seemed a worse candidate to make a dramatic improvement.

Let’s start with the roster moves, or lack thereof. Given their off-season inaction, it was hard to imagine the Sonics improving at all. The team’s only major transaction was a subtraction rather than an addition, as sharpshooter Brent Barry left for San Antonio. Their one free-agent signing, Turkish swingman Ibrahim Kutluay, failed to score a single point before being cut on February 1.They got no help from their rookie class either, as high-schooler Robert Swift wasn’t ready to contribute. The only off-season maneuver of any significance was the trade of Calvin Booth to Dallas for Danny Fortson.

What’s more, most of the players had their bags packed. Seattle’s biggest star, Ray Allen, was entering a contract year and couldn’t work out an extension, leading many observers to believe that he’d be traded by mid-season. He wasn’t the only one on a month-to-month lease, either, as eight of the 13 Sonics under contract were in their walk years.

The uncertainty extended to the staff, where coach Nate McMillan’s contract was set to expire after the season, and nobody was clamoring for him to hang around. That was a bad omen: Nearly every coach in this situation (Byron Scott being a local example) has been tuned out by his players and canned by mid-season. General Manager Rick Sund was in the same position, which limited his ability to make guarantees to anyone else on the roster.

Thus, a Sonic meltdown was an easy scenario to envision: The free agents would play for their own stats, everyone would ignore McMillan, nobody would play defense, and the organization would blow the team up at the trade deadline.

What’s happened instead has been utterly remarkable: Not only do the Sonics not stink, they’re a serious threat to make a deep playoff run. Since they’re doing it with the same cast that was so beatable a year ago, their improbable rise has baffled the experts.

The lack of a credible explanation is partly due to the press relying on tried-and-true formulas for surprise teams like the Sonics: Nearly all of the credit for a team’s improvement goes to primary off-season acquisitions (such as Steve Nash in Phoenix), whether or not it’s deserved. But Seattle’s only addition was Fortson, who plays fewer than 20 minutes a night, so the formula doesn’t fit here.

The next item on the pundit’s checklist, the Rising Star Lifting His Team theory (see James, LeBron) doesn’t apply either. Allen is hardly a rising star at 29, and his numbers are actually down slightly from a year ago.

In fact, the turnaround can’t be credited to any of the primary players. According to my per-minute rating of statistical performance, the Player Efficiency Rating (PER), the five Sonics who play the most minutes – Allen, Rashard Lewis, Vladimir Radmanovic, Antonio Daniels, and Luke Ridnour – have seen only minor statistical improvement this year; enough to improve a team’s record by a couple of wins, but certainly not by the quantum leap that Seattle has made.

So how is the team so much better? Just look to the bench. While the five Sonics with the most minutes haven’t improved, three of the other four players who get consistent playing time have provided a massive improvement.

Fortson, for one, is enjoying one of his best seasons. He’s always been a great rebounder, but this year he’s scoring too, averaging 18.7 points per 40 minutes and shooting 53.9%.Another forward, Nick Collison, missed all of last year with a shoulder injury; this year he’s averaging 13.2 points per 40 minutes and shooting 53%.And Reggie Evans, a non-scorer a year ago, is at least scoring enough to make himself relevant on offense. That’s all he needs to be a valuable player since he and Fortson are the league’s two best per-minute rebounders.

The impact of those three has been enormous if only because the Sonics’ big men were so pathetic last season. Replacing Booth with Fortson, Vitaly Potapenko with Collison, and the old Evans with the new Evans has given the Sonics a much-needed facelift in the paint. The PER improvement from this season’s trio is nearly 10 times that of the five most used Sonics. Thus, despite the limited minutes, their contributions to the team’s improvement have been much greater.

To see the impact of the Sonics’ frontcourt trio, look at the team’s rebounding numbers. Last season, Seattle was the worst defensive rebounding team in basketball, pulling down just 68.5% of opponent misses, and ranked fourth from the bottom overall. This season, the Sonics are the fourth-best overall and rank no. 1 on the offensive glass.

So while the pundits have focused much of their attention on Seattle’s brilliant shooters, the real difference between this year and last is what happens when a shot misses. Thanks to the massive frontcourt upgrade, Seattle can bang with the big boys in a way it simply could not last season.

The mystery of the Sonics’ improvement should no longer be keeping experts sleepless in Seattle, or anywhere else. Fortson, Collison, and Evans aren’t close to being the biggest stars on the Sonics. But it’s their dirty work that’s allowing Seattle’s stars to win the games they were losing a year ago.