Whenever I'm watching someone play piano on a tutorial or something I get annoyed when they play with 'flair'. Like the exaggerated hand movements and stuff comes across as so pretentious to me. Just play the piano! 
Could be I'm jealous of them?
I don't know if this is exactly what you're talking about, but I have thought the same about singers using a bunch of flair and exaggerated / weird head or hand movements. Then I started singing and realized you can't really control them. When you are totally locked in and have complete focus on performing, the face and body contortions and weird hand motions just happen, and somehow they help you hit the notes, I can't really explain it!
Idk if piano is the same because the hand flairs do seem more performative
I know. When I'm together with some friends having a sing sesh, they call it my "O face," but I can't help it. I joke that I must have hit a high G, a.k.a. my G-spot. All joking aside, you truly can't control what happens when you are lost in song.
Similarly, I find myself dancing while standing and playing guitar. But like swing dancing, not dancing in place.
