@bb, aah, I see now.

Maybe you are right about what you said.

You see, each dot on this graph represents the average difficulty of 9 questions for a total of 4 dots.

so if the average difficulty of 9 questions starts at close to medium and then remains at medium-high, then it absolutely must have had some easy questions in there as well.

So you are right.

But I am still not able to understand how the gmat could give someone a 99 percentile by testing them on equal proportions on easy, medium and hard questions?

The graph skews medium-high for on this so surely there has to be more difficult questions and less easy/medium questions? What are your thoughts?
I can tell you from experience, at least in the verbal section, if you are scoring anywhere near the mid 40s, you will not be seeing an easy question, and in my experience, you will see some more rare question types.
Moreover, like bb said, it is much harder to make a difficult question, and the gmat gets a new question pool every 16 days or so. So by that reasoning, there will probably be more 650-700 questions than 700+. In the GMAT, becasue of it’s adaptive nature, it is somewhat more important to not mess up easy/medium questions, than to get the super hard ones correct. Missing a super hard one, might drop you from say Q51 to Q50, but missing an easy one will ruin the rest of your exam.