A factor in those stats is minutes, isn't it?
If you look at "per 36 minutes" stats, Lebron is at 26.8 points per 36. Pierce is at 19.7, KG is at 20.7, and Ray is at 17.5. That means, per 36 minutes, the Big Three is scoring 57.9 points, or 31.1 points more than Lebron. That gap presumably only increases as the players get more minutes.
A lot will have to do with how Doc manages his rotation, but if this is a tight series, I expect the big three to all average around 40 mpg. In that case, I fully expect them to outscore the Cavs' top three starters, and I like Rondo matched up on Delonte, as well.