She was raised to be like a male by Magyar [x] and was surrounded by males as she matured. She had no females to compare her physiology to either. It was only until her physical differences (i.e., her breasts, strength, and stamina) become more noticeable that she recognized her sex.
(I’m less inclined to believe the whole difference in strength and stamina aspects, because it’s stereotypical, but that’s the logic used in the manga).
Ellawritesficssometimes 