I guess most people's gaydar, when used on men, operates on signals that tend to be read as feminine or effeminate. So trans women who are still living as men will often be read as gay. And gayness is much more in the public eye than transness, so most people will think "gay" before they think "trans." Or, for that matter, "straight." Because, you know, straight men NEVER come across as gay.
I often wonder about the origin of those signals in gay men. There seems to be a higher percentage of effeminate gay men than effeminate straight men. But do they start out that way? How much is inherent? How much of this apparent effeminacy is openly fostered by gay communities and assiduously rooted out by straight communities? How many straight men come across as gay but aren't? How accurate is gaydar?
Sorry, now you've got me going.