I think quite a few people, even well-meaning ones, say "LGBT" and think of gays and lesbians only by default. They don't even parse what the letters stand for.

Saw an interaction where a person contrasted LGBT people against straight people (so, no straight T people, huh), and the other told them "don't forget about bisexual people!".

Uh, they didn't? P. sure the B was in there already?

So, TWO ppl, both pro-LGBT, seemingly used the acronym with implicit assumptions that're just... off.