I have a mathematics degree as well :-)
The answer is simple - since some players can be removed from a tournament during creating the sections, it is not possible to determine how many slots you will really need BEFORE the sections are generated. That's why there is the 5/10 slots rule. The second reason is to give pawns another reason why to upgrade to a higher level where this limit is no more applied.
Shortly - you are playing here for free, so you must accept all pros and cons. I see no reason why I should add more advantages to non-paying members.