ehhh... tbh, there is validity to that but it is more of a generalization. Some students say the later exams (9-11) are more representative. I think this is because they provide a certain "weirdness" (feeling of unfamiliarity) with the science passages . I am under the impression that AAMC makes the current MCAT in a way that tries to catch students off-guard. There is absolutely no way to predict what you will get on your given exam. It demands critical thinking and an ability to adapt to whatever kind of passages you are given (some MCATs will have more experimental passages than others.... some will have more orgo than others... some verbals will start with hardest[or easiest] and on and on).
That being said, I wouldn't say questions are strictly harder on current MCAT. The bio section on the July 2 seemed on same level of difficulty as AAMC 3 bio section, but the physics section felt more like the latter AAMC's (in fact, there was even more "weirdness" even though the difficulty level wasn't terrible).
To answer your question, you should expect on the MCAT the unexpected. Sounds paradoxical, but the way to do this is to keep taking practice tests. Don't really focus on the number "31" (that is just a scaled estimated score). The more important thing is to track progress and work on timing and why you are getting certain questions wrong. Don't doubt your ability to increase above a 31 (some top scorers start even lower).