Well, when you put it that way, you do make sense(all of you). But you have't yet talk about yet the fact(or implication) that is brought up in the second movie, also in the trailer,(not really a spoiler). The three girls that are the main antagonist were banish to that world. Meaning that the one who sent them there must have known that there was not a human version of them there originally. With that said she could not really have a human version.
Besides I don't think that some of the humans match age with the ponies, just one I can think of... Photo Finish: pony is an adult and in her line of work.... human version is still in school. With that also said the two worlds are not identical in terms of pony to human counterparts.
(wow that was a mouthful)