To sort of put a face on one of your points, the celebrated Marsha P. Johnson of Stonewall fame would not have been counted as an adult transgender person in surveys or polls, because she did not identify as transgender before she died in the early 90s. Nor did many of her contemporaries, who like her would most probably identity as transgender today.

I ran into Marsha a couple times at the Greenwich Village LGBT Community Center, and I knew people who saw her much more often than I did. She presented as a woman all the time in public and always used female pronouns for herself.

But she wouldn’t count as trans in polls. Polls can only capture so much.