I would imagine that it is because the hole for the thumb (at the top) is longer than the hole for the fingers, and the longer hole is required because the thumb points at a different angle (approx. North West) to the fingers which point approx. due West, assuming that the scissor blades are pointing due North. If you see what I mean??
No - I got the question quite wrong. I was cutting my fingernails this morning. Scissors in right hand, easy to cut nails on left hand. Scissors in left hand, impossible to cut nails on right hand.
Have to contort left arm to turn the scissors round and cut in the opposite direction. They weren't upside down at all. My fault!
The reason is because scissors are designed to be used in the right hand.The action of closing the scissors forces the blades together as they meet each other. If you use your left hand, you tend to push the blades apart. Left handed scissors are made to cross over in the opposite sense, so your left hand pushes the blades together. If you try to use left handed scissors in your right hand, they will not work effectively, for the same reason.
Scissors are surprisingly difficult for us lefties, so are other normal everyday houshold appliances and gadgets. Try using a potato peeler- almost impossible.