WoTC isn't claiming to be exact. They are claiming that they took those percentages that answered and used those numbers to come to a general population opinion. Of course it isn't precise within 0.1% of the truth. That was never their goal.
How do you know it isn't the middle? Do you think it is possible for something to score a 0%? How about a 10%, think that is a possible score?
If I gave you the following sequence of numbers, 81, 82, 83, 84, 85, 86, 87, 88, 89 what do you think the average value is? Is it 50? Does that make logical sense?
Just because you ignore their definitions and process doesn't mean their process isn't working. No teacher is every going to happy with a class average of 50%, no matter how much you insist that would be a true average result and that a 70% is completely bonkers and innacurate to real education.
False as has been stated over and over and over and over and over again.
Sure, you could do it that way. But that'd be a dumb way to do it and would lead to bad results, and that isn't how they are dividing the categories. Stop insisting that the only possible way they could do it is your way. Especially when you are clearly working on such a limited amount of knowledge on the subject.