I think the author is saying that if there's no variance (e.g. all schools have 10 students), then our school is as big as all the other schools, and as you increase the variance, the odds of you being in a larger than average school increases. But no matter the distribution, we're never most likely to be in a smaller than average school.

You cut out the context in some of your quotes that makes this clearer. The article says

> imagine that planet populations are distributed in the same pattern as countries on Earth ... in that case, most planets will have a very much smaller population

They're saying, for purposes of illustration, imagine planet populations were distributed like countries on Earth. But that's not the distribution they use in their actual calculation. It's just to help explain.