Hard to say since I don't know what I got wrong. In my score report, I didn't star some subjects that I starred in 7 practice NBMEs, so I think there were some flukes. There were 3 questions I 100% knew the answer for, but was stupid.
My test didn't test anything that was completely foreign, so maybe it had a harsh curve. All the really weird/hard questions I got right, so if there are experimental questions, I probably got those right. The 13 I know I got wrong did not seem experimental.
NBME 19 was the most predictive test for me (I scored 250).