What's the reasoning for not mouthing questions while doing the GMAT in person?

I get online - the proctor might assume there's someone in the room with you maybe who is reading your lips and helping you I guess? But in person? If I'm clearly mouthing to myself, what's the issue there?
I ask because all my life, the only way to get around ADHD is to either listen to white noise or mouth what I'm reading so it gets in my brain - and it's never really be an issue. Would I really be penalized if I had to mouth the VC questions to myself? If so, I'm going to have to stop ASAP lol