I think this demonstrates just how much it's a combination of everything. I mean, every day, you will see women who in some way, aren't 'all feminine'. Yet, we don't for a moment think that they're female impersonators. Why?
Because even when they display a masculine 'tell', the rest of them is 100% female.
We don't have that. One little 'tell' and it invites further inspection of us, and very, very few can pass that.
We simply don't have a lifetime of being female. Even when we practice, at some point, we're going to over do or under do something, or a simple physical anatomical difference, and that becomes the dead give away that maybe we're not exactly what we seem to be.
Then the scrutiny starts, and BINGO! We're 'made'.
And it all takes as little as a second.
For most of us, this is due to our continued life of living as a male. When we crossdress and try to present as females, we're just acting the part. IRL, We keep all our male behaviors, so the moment our guard is down, we just slip back into male mode, and it only takes a couple of seconds of this to broadcast to the world that we're not women. It's just a normal subconscious thing to do, behaving naturally, that gives us away.