Their defensive ratings and statistics are similar to OKC/LAL, why do you see such a difference Who?
Interior defense
DeJuan Blair and Matt Bonner are two of the weakest defensive PFs playing regular minutes in the league. Tim Duncan is a good but unexceptional defender. Splitter a solid but unexceptional defender. Lack of foot-speed. Lack of rim-protection. PnR defense is vulnerable. Defensive rotations aren't quick enough.
Boris Diaw is their best defensive PF but he looks only about average this year due to all that excess weight he's carrying. He's their best hope for tightening up that defense some.
In contrast, the Thunder have Ibaka (high level defender) + Perk + Collison while the Lakers have Bynum (best overall defender) + Pau (comparable to Duncan defensively). Both teams have far more defensive talent than the Spurs.
The Spurs perimeter defense is well above average though. Danny Green and Kawhi Leonard. Ginobili. Parker. Stephen Jackson. A lot of good weapons.