Hc that since Arthur basically always wears his hat when around others, he uses it to conceal/hide his emotions by tilting his head down so his hat covers his face.
Except he's been hanging around Charles so much that sometimes he's caught without his hat for whatever reason. So when he tries to hide a smirk or a frown (or a blush for my charthur shippers) he instinctively looks down forgetting he doesn't have his hat.
Anyway Charles is observant and picks up on the tell super quick. He can tell when something's up bc Arthur hides behind his hat (or the absence of one).
Well Arthur isn't even aware that he does this and is equally curious and terrified of how easily Charles can read him. No one has ever been able to pick up on his small emotional cues or if they did they didn't mention it while Charles knows almost immediately. Arthur probably thought at one point or another Charles had mind reading capabilities because it is the only explanation he could think of.
And the whole time Arthur thinks he is good at concealing his emotions when it's really just his hat.
