>>16760455The "U" in "RU" or "OU" means "Used."
They're not decided by some kind of arbitrary power calculator, they're decided by usage percentages.
OU pokemon are generally considered ones that fit well into most team dynamics you need them for. UU and RU are not BAD, but they either require very specific synergy or mastery of a specific trait they possess.
For instance, if you're running a sun team and want a little extra bulk, you can use Rotom-F, which is tiered lower. You go in with the understanding that it'll have more weaknesses than Rotom-W, but at the same time it fits your synergy better since its fire move is strengthened while Rotom-W's is weakened.
And that's not even getting into some of the creative things you can do with things like the Eeveelutions
