Yep. It is pretty much totally random. For one-day substitutes, you don't need to be certified to teach anything at all. They don't care what you are best at. The idea is to get a basically responsible adult body into place.

In their defense, generally these folks have two hours to find subs for the entire district. That's two hours to make... 40 to 80 phone calls to fill all the slots. They don't have time to match up specialties.