It depends on what you're dreaming about when you wet the bed.
I sometimes will wet the bed if I happen to be dreaming about being near a waterfall or sometimes I'll be dreaming that I'm sitting on a toilet and urinating and then I suddenly wake up and realize that I was in bed while peeing.
It's our minds playing tricks on us and it's very common to be dreaming about urinating and then end up peeing in your bed.