Left-handedness and Math

It's trivial, but still, I've never come across the thought.
Grandma is practicing Math with my brother. He's not stupid, but he is lazy.
But grandma told me (just by the way), another problem of his, though of perhaps minor importance, is that he writes the expressions (left to right, of course) and he covers them by his left hand (as he's a left-handed) as he writes on. Certainly it is not the source of his Math problems, but surely a nuisance.