I think it mostly boils down to a matter of preference.
For some it sounds better to say "She's the strongest woman on the planet!" than "She's the second strongest being on the planet!".
In the case of Superman saying "He's the strongest man in the world!" is redundant as he's widely regarded as the strongest of all, man or woman.
Sure, one could use the gender neutral "She's one of the strongest beings on the planet!" but that's relative enough to take some of the shine away from her as an individual.