While it’s hard to imagine anyone more uniquely unqualified to answer this question (unless maybe it’s Jayne up there), I do have a theory:
I think it has to do with the position of the head. Women crane the head forward to bring the head closer to the mirror. This stretches the throat, creating a tension that’s relieved by opening the mouth. Now slide your head back to a neutral position and you’ll see that your mouth closes easily.
You ca see this for yourself by reaching far forward with your chin. The mouth naturally wants to open to relieve that stretch in the throat, and it feels a bit strained to keep it forcibly closed.