The MCAT isn't like the GRE, where the level is supposed to build as you get the easier questions right. There is no intention to design either Kaplan's or the AAMC's tests in that way.
Some people feel the end of a test is harder because they tire out, and some people worry more about timing at that point so they feel more rushed and it feels more difficult. I teach for Kaplan, and every time there's a practice test I hear both sides--as well as the whole "Kaplan is harder than AAMC/Kaplan is easier than AAMC" controversy. Also, different people find different passages more challenging--in PS, some people would find a titration passage difficult but an electrostatics one would be a breeze, and vice versa. It just depends on your strengths. I've concluded that at least 90% of all these "differences" are totally perceptual.
Or such is my perception. 😎
