Supervisor at work calls me by my short form of name.... any clue why?

Bring a $100 Million sexual harassment suit against him? Lol

When I played sports in high school the coach used to shorten my last name and call me that. People usually call me by my full name (Jessica) but sometimes I get Jess or Jesse. Usually it is people that know me pretty well that call me Jess.
If it bothers you, make it a point to say something, in a non aggressive manner, to let him know you don't want him to call you that anymore.
like every time he does, go back and say oh I'm sorry, my name is _____, I didn't realize you were talking to me. Or make up a story about how your grandpa used to call you that before he died (I hope that isn't true, I am just trying to give you some ideas) and it makes you sad every time some else does it.

I have a bunch of guys at work that call me all kinds of things. There are a bunch of guys from other countries so they use there own way of shortening names. I have been called Jessicita a lot. One guy always calls me that. It doesn't bother me, I think it is cute. A sign of friendship.

It could be that he knows someone with the same name and is used to using that version of it, or it could be that he thinks the long form is too formal and wants to be casual and friendly with you. I wouldn't read too much into why he's doing it, but if it makes you uncomfortable, just be polite and honest about it. Next time he calls you by the short name, just say, "Actually, I prefer ______" and I'm sure he'll be fine with it.