I'll help you understand it, don't worry.
Average means that you take a sample and find the number in the middle.
So if you have 10 characters with some arbitrary values that sum up to 80, the average is 8.
All characters that fall below this number, are below average.
All characters that fall above this numbers, are above average.
So if the average character in the game is let's say D cup, those with B caps are below average, those with E cups are above average.
Let's say that the minimum is 1, and the maximum is 12
A possible naming scheme is
Small: 1 to 3
Below Average: 4 to 6
Average: 7 to 9
Above Average: 10 to 11
Top: 12
Depending on the context and how the characters can be grouped, a different naming scheme can work.
In the post you quoted, the characters are grouped because their boob size is roughly equal.
So even if Clover is bigger than Devana, it doesn't make much sense to put her in a different group,
because the smallest of the next group is significantly bigger than her.
This is a different exercise to the previous one. You can attempt to define a Superhuman based on how many humans he would kill, or count how much force they can generate based on the average MMA fighter punch, or whatever.
The Superhuman average will still be thousand of times more than the average Human.
So if you try to make a tier list on how much weight Superhumans can bench, and the Human average is lets say 100kg, you can't possibly claim that those Superhumans who can lift up to 100kg are Average.
This is exactly what you did with the boob size.
So no, your premise is flawed beyond any reason. But no hard feelings.