My overly used reply to the stats argument has always been the Brunt > Xavi one from the 2010/11 season. More goals and more assists in a worse team, so he's obviously a better player. Or not. As Lenny and Spurf have rightly pointed out, they're a useful guide at times, but can often be either pointless or misleading. I think that the biggest improvement in Defoe's game has been his attitude. Maybe it's because he's first choice in our side, which hasn't often been the case, but he's shown a willing to learn, change and really work on his weaknesses. He seems a little more relaxed too, but in a good way, as he's not snatching at chances or trying to force things that simply won't work in a desperate attempt to impress and keep his place.
Defoe has just been unlucky, in the sense he has largely spent his career with us not viewed as the main man, even though he has had periods in which he was first choice, as fans we have never fully appreciated him and always felt we could do better. I would say we have all been guilty of this. However as you mention about him now not being desperate to impress and keep his place, he is now viewed as our leading striker, by a manager that views him as the 1st name on the team sheet, so it shows just how important it is, to have a manager that believes in his ability.