One of the first things that the original 13 colonies did was form Militias.
So, why'd we put 13 stars on this beanie and call it "Thirteen"?
I don't really know for sure.
But do you really care? As long as this beanie is keeping your head warm and dry and you look pretty badass wearing it, right? Would you even notice if it had 14 stars? What about 12? You just counted them, didn't you?