When I was a middle-school kid trying to learn D&D, I just foolishly assumed the math was deliberately obtuse, to make sure that only "older kids" would be able to play it. The way I saw it, that complicated math was intended to be a rite of passage (at best), or a bar to entry (at worst). So we made up our own math and our own rules, as kids will do.
But now I'm an adult with an engineering degree and I can confirm, yes, that was probably the case. And our solution is still the best recommended course of action.