San Antonio's only 20-18 when they don't hit 7 3's. But this means if they do hit 7 they're 36-8.
But that's not all.
San Antonio is 40-7 when they get 21 assists. If they get less than 21, they're 16-19.
But there's more.
If San Antonio gets their optimal game and have both 7 3s and 21 assists they're 30-4. If they get neither 7 3s nor 21 assists, they're 10-15. If they get 21 assists but not 7 3's they're 10-3. If they get 7 3's but not 21 assists, they're 6-4.
Just a little more.
So they break down is like this.
And a little bit more
The Spurs 3's, assists, and wins are all tied together. When they are passing the ball well to their three point shooters (thus 21 and 7), they're pretty much unstoppable. They're also good (though not as good) when they can just do one of the two, but it's rare that they do only one of the two (a little less than 3 out of ten games). This means that if you can stop one, you can likely stop the other.
Our perimeter players have to stay at home on the Spurs 3 pt. shooters. The Spurs offense heavily depends on the three pointer. In fact, only a few teams take more threes per FGA than the Spurs. The Suns do. But unlike the Spurs, when they Suns don't get their median assists (27) and their median threes (9), they still play well (15-8). This is because we have several players who can score without the three (Nash, STAT, Shaq, Diaw, Barbosa, Hill, Giricek) but the Spurs have only their big three.
Thus the Spurs are extremely vulnerable if they lose the three point shot. And judging by their individual stats, it seems they likely get many of their three's from Duncan double teams and Parker/Ginobili kick outs, since those three players give out almost all of the Spurs assists.
So as long as we do not double team and only rotate our big men to a driving Parker/Ginobili, Udoka, Bowen, and Finley (and Barry or Horry if they play) will be useless (all three depend almost entirely on threes for points).
Thus not only will Parker, Duncan, and Ginobili all have to have solid games, at least two of them will need outstanding games. This is unlikely, and that is why the Spurs lose most games in which they cannot dish out to their spot up shooters.