That is a fascinating question to me and I have long considered the answers to be a mix of our learned behaviors and, in our case, from the changes in our physical being after HRT etc.
So many of us worked way too long to "stop acting like a sissy girl" as we learned how to man up. One of the first lessons, I recall, was in grade school where the boys laughed at me for throwing a ball wrong and being "afraid" to catch a stupid (american) foot ball. It really seems a dumb shape still

I remember countless questions along the lines of why do you act like that. An SO asked me a few years ago why I was "walking that way" and i was unaware of my mincing steps? For me/us this is not pretentious behavior, it is something more deeply inherent. But from where and why? Those eternal questions bring us back to the dissection of self along the nature and nurture quagmire.
Early transition for me was a profound relief as I did not appreciate how much effort it had been to unconsciously guard my behavior and not "talk with my hands" etc. Now I sort of go the other way and don't want to fall into the stereotypical boxes that suggest there is one way for women and another for men to walk, talk and live.
Allowing ourselves precious freedoms to feel and be just you and me is an ongoing sense of finding our way in family, community, culture and love. Tis a grand journey with unique perspectives as transgender people.