It looks like the data here are an average across several seasons. Since the NBA doesn't do a truly balanced schedule this would probably fluctuate (just a bit) from year to year based on the actual schedule, though that's a minor point. As Warhawk said, this isn't terribly surprising given the relative density of teams (and population) in the eastern and western portions of the continent.
On the other hand, we'd fare pretty well in average game-day temperature because we play 47 games in California. Sure, the games are played inside, but you've gotta go out in that stuff to get to the arena! Brrr!