I had one day about a month ago where I was called Sir 5 times in 1 day. I wish I had stayed in bed that day. And I've been full time for over 2 years now. I went from this:

to this:

I can kind of understand. I know I was pissed that day I was misgendered, but it just made me work harder at passing and certainly not take it for granted.
Anyway, I'm not going to guess as to why you get misgendered because I don't exactly have a picture that tells the whole story. I dont know your voice, mannerisms, walk, etc. But from personal experience, it can and does happen. and you're not the only one who gets frustrated. *hugs*