My brother once asked me what the difference was between "bi" and "pan," and I told him, "there isn't one, it's just about communities."
"But there has to be a difference, because bi means two and pan means all."
"Oh, people will tell you there's a difference. People will say one or the other is reductive, or maybe they'll say something like, bi people can have preferences among multiple genders whereas pan people have no preference. But you can't prove every bi person or every pan person feels a certain way about something, and I've met enough to know these 'tests' aren't reliable. It's just about the vibe and community you prefer. Some people pick based on the flag, or whichever they learn about first."
He didn't get it and insisted there must be some fundamental distinction.
...Anyway, I feel the same way about "transsexual" and "transgender."