If you check the Official Guide for GMAT Review (2015 or 13th edition, 12th edition, etc) you will see that there are many as 8 questions for a single passage. Clearly 8 questions will not be given to a particular test-taker for a single passage.
You will also notice that there are pairs of these questions that both ask for the same information, but they do so in different ways. One of the two questions is usually noticeably more difficult.
So you see that the computer has many questions to draw from for each passage that you are a assigned. The overall passage might be assigned to people who at that point are estimated to be from a 680 to a 720 and then the questions you are given will refine that difficulty level.
So the answer to your question is that each question does have its own difficulty level that contributes to the total difficult of the passage and by selecting different questions for you the difficulty can be adjusted.
Now one more thing to remember is that the test does not adapt to you in the middle of a reading comp set. All of the questions are selected when the passage is chosen. So if a particular passage has 5 questions you will receive the same 5th question whether you get all of the proceeding 4 questions right or all 4 wrong or some right and some wrong.
hope it helps!
