I'm ambidextrous. I also know how this came to be....
If I would have had no outside influence, I would have been right-handed, but as a kid my mom used to get behind me and taught me write and eat left handed as she was left handed and therefore taught me to be left-handed at those activities.
Now, I do some things left handed and some things right handed. When it comes to doing new activities (jobs and such) I tend to use whatever hand I was taught to initially.
It's all very confusing actually because some things I can do left handed and not right handed, some things I can do right handed but not left handed and some things I can do with either hand. There seems to be no rhyme or reason as to which hand I'll use for what task. So I just say I'm ambidextrous and leave it at that.
It's one of those questions that gives me pause whenever I am asked about it.