Maybe you were having a bad day?
I don't mean any offense but judging by user's experiences posted on here over the past couple years regarding trends in their NBME scores and their actual scores, you can see a pretty strong correlation between their NBME and actual scores.
It is unfortunate in your case that your actual exam, for whatever reason(s), didn't pan out the way NBMEs had predicted. However, good majority of the people have had their actual scores accurately predicted by the NBMEs.
Again, it sucks to hear about your experience. I wish you best of luck on step 2.