a question about voice

ive been thinking for quite a while now about voices and how they come to be. my understanding is that when we are young, the voice of a child amab and a child afab is the same voice the only difference is in how it is used

this leaves me with a question though. if someone believes from a young age that they are the opposite gender, why are their speaking mannerisms often still colored by the sex assigned at birth rather than the true gender?

i suppose i have some level of personal investment in this question, as someone who has given up on transitioning, including voice training. i know for myself that i enjoyed a lot of girlhood in spite of the sexism, and i didnt feel any desire to make myself any less feminine until around age 11 when my chest first started developing.

i guess im asking this because im curious to hear what you all think. why do we pick up these speaking mannerisms, even when they dont align with the gender we desire?

one last thing. reddit is telling me to flag this as nsfw, so i am doing so, but i dont know if mentioning a fact of my puberty (my chest beginning to develop) is actually nsfw