You should have a third option for ambidextrous. Because of what I was taught, I typically write right-handed, but there are a lot of things in which I use left for. I can use both hands for most situations excluding tennis and writing.
Oh...and Emily, the reason is probably because most computers have the mouse set up on the right side, and you became accustomed to using computers that way. I have a friend who is completely left handed in everything but mouse use, which he learned because mice are standardly set up that way.