To be honest with you I don't have a reference for that. I was told this by University professors.
The NBME does not disclose the way they select questions and blocks for each candidate but there's strong argument saying that block difficulty varies and interactive.
I talked to three recent students who just took the exam. All of them told me that the first few blocks were far more difficult than the blocks at the end of the exam so that must have something to do with this speculation.
The phenomenon is called "
Computerized Adaptive Testing" and it's not mentioned in the official USMLE bulletin but many believes it exists such as
this page.
However, the NBME website mentions that they are currently researching this possibility see here
http://www.nbme.org/programs-services/health-professionals/special-resources.html