Quote from padware
The leaderboard update happens when you exit the world on PS3 at the moment, so if you are checking the leaderboards while in the world, you won't see an update.
We'll change that in future to update after a certain time has passed when you are in a world, as well as when you exit the world.
This doesn't seem to be the case, as far as I can tell. None of my statistics update, even after leaving the world. Also tried waiting overnight before returning to the world, and still no leaderboard update.