Naturally, his back is a place he's not particularly familiar with how it looks. There are mirrors, but he doesn't always have access to one and, when he does, he doesn't care to study his back to closely. So he doesn't think that Luke's doing anything but washing it, really, until he speaks.

"Oh, yeah." Of course there would be plenty of them. "I'd offer to tell you, but I don't think I remember most of them."

He's been hurt so many times, he really only remembers the very bad ones.