That was actually a bug once. Remember the "super-AAA" hitters? They were all from the Federal League, American Association, Players League, etc, and they all were normalized against .000 batting averages, 0 HR/Ab, etc...
They generally batted .400-.500, despite a .250 average (or so) in real life.
I was the guy who identified that bug, spotted the pattern. If I recall right, they now normalize against most-recent year combined MLB average.
I may be wrong about the MLB. But i think that was how TZ described it was working, and if not, it is very close to that. Certainly, the "Super-AAA" hitters are dead.