I think it's actually to do with whether the cutting edge is visible or not. Right-handed scissors held to the right of your eyes have effectively no margin bet

I think it's actually to do with whether the cutting edge is visible or not. Right-handed scissors held to the right of your eyes have effectively no margin between the cut and the edge of the blade. Right-handed scissors on your left (or vice versa) have a blade-width worth of margin between what you see and what you cut. Funnily enough, you can also see well if you cut with right-handed scissors on your left side, but with the paper held above your eye-line.