I know Durant is more efficient when you look at the numbers, but if anyone was watching the game @ OKC you can see the partial reason for such a difference.
Melo got 9 offensive boards, constantly battled under the rim and got hit A LOT of times, yet didn't go to the line until the last 30 seconds of the game, Durant on the other hand shot 15 of them and a lot of those were soft fouls to say the least.
I don't know about you, but having watched a lot of games (especialy Knicks' games, but also quite a few OKC ones) I know for a fact that the refs have a very different "relationship" with Durant than they do with Melo, it helps getting to the foul line when you mess up a shot and not missing a shot because you were fouled (but there's no call), I know that's not the whole story and I'm not even going for it, just trying to mention the obvious (well, it should be to those who don't hate Melo/are in love with Durant)