When you look at these stats, the 4 best defensive teams are in the eastern conference. At the same time 8 of the 10 best offensive teams are in the western conference.
So:
Are EC teams better defensively or are they just facing weak offensive teams in their conference?
It's likely a little of the both but WC teams are 102-78 this year against EC teams. It could lead to think that a huge reason why these EC teams have such good defensive stats is the low level of their conference.
When you compare Spurs defensive stats only to WC teams, they look way better.