I once had three variatus. The first were a male and female pair that just flitted around swimming like they do all the time. Then came another spunkier little female who mostly moved around by moving her tail in a sine wave like they do to hover, just using all her fins this way - a markedly different way of swimming that I think was just because it was her personality. I watched her and the male interacting when they first met. He saw her swimming this way and decided to try it.
All three platys were swimming her way by the end of the day.
My point is that fish are very expressive with their bodies, especially the fins and how they move. So there could be any number of reasons. Just look at her personality, try to see what you can.
Or you could post a video : )