The test has no infinite difficulty questions so they do plateau.

If you keep answering them correctly, the questions in a certain topic will plateau to high difficulty. If the next question is from a different topic, the test may start probing from middle level, to establish your proficiency in that topic. If you answer it correctly the next question in that topic, whenever it happens, will be higher level untill that topic plateaus at high difficulty too.

Remember though, difficulty is completeley relative. You might think probability is a piece of cake, but the next guy wants to wring the test maker's necks. The plateau is completely indistinguishable for us as test-takers because they'll jump from geometry to exponents to factoring to rates, and the content of the question can make or break us!

They only ask a certain number on questions on each math topic (3-4) so when they exhaust that topic they go on to the difficult questions on the next topic. Thus, the difficulty only gets harder to a certain point. I think they have about 3 difficulty levels (medium, moderate, and high).