With Nadal, we know more than twenty of those were versus Verdasco and Lopez combined. He's generally superior to them, and I think there are added mental forces at work there.
Others are right that Fed's numbers are skewed because
It's interesting, really, aren't Roddick and Hewitt also among the top five of the Open Era versus lefties?
A quick search says so:
Most other names are expected. Fed sits at #9. Murray, perhaps surprisingly, sits as high as #13. I don't see Djokovic on there.