here's my theory about why NBME two is considered the best score predictor, if EVERYONE here keeps telling everyone to keep the NBME 2 till the very end, then everyone does form 2 after they've studied and since the scores are a reflection of the mean of the test takers, then it would make sense that the one that most people are taking when they are most prepared is the one that's the best predictor...
i've just spent the past few hours studying biochem so my mind is fried, so if this makes no sense, please lap it up to biochemical encephalopathy