Teaching the wrong subject

At the last school I taught at, I was the resident biology teacher. Actually, I'm a physics / mathematics major. The"physics and chemistry teacher" at the school - a biology major. The math teacher is also a biology major. What's up with that? This school is considered one of the best in Montreal, yet this discord occurs. (I would have loved to trade classes with the other teachers, as I really know very little about biology.)

This happens a lot, in many schools. For instance, my partner teachers, back at university, were hardly math majors (one was a geology major, the other an industrial art's teacher, and yet another a phys. ed. major). From year to year, teachers usually get to say what they would prefer to teach the next year, so why does this discord not adjust itself? Why do some teachers not like to teach their own prefered subject?

Or maybe, there are not enough math and physics teachers out there, so other majors need to take over. Hmmmm. Does this happen with other subject areas, or just the sciences?