I don't believe in stats like that - scoring by position is sooooo unrelated to the actual defense played. It's a combination of factors - for example, Fish will get burned by his guy and saved by Bynum/Pau at the rim making plays. That doesn't show up in stats, but the penetration given up is the problem. Kickouts to shooters also a problem when the PG is burned. You get caught up in switches a lot, especially by design, to create mismatches.
I've been watching Sessions for a couple months now when I can since we've been looking at him, and I've watched Fish forever. Sessions is a better basic defender than Fisher (not hard) and doesn't give up penetration nearly as easily. The stats test is one thing, but the eye test is golden.