Can you count? Do you know all the math signs and how to do PEMDAS? Then you know all the things you wouldn't otherwise know without a guide or a calculator.
Need to instruct a student who asks you a stumper? You just need to know how to formulate and can use a guide or a calculator to help you if you had to.
Not good at math without one? You could chalk it up to everyone having days like that, as it's not like you could expect a school to remove you because of brainfart.
If you became any other kind of teacher (English, history, science, etc.) you would need to build up a starter pack of knowledge on the subject, such as different obscure forms of grammar or what happened during the war of 1812. With math, you aren't expected to be perfect since it's a circumstantial mental skill, and when you subtract that from the equation (no pun intended), you can have any skill level that allows you to pass the instructions on how to perform equations to the students.
You also don't have to know what everything means, since there are things that we take for granted only advanced math classes will teach you.
That's funny, I thought it was mainly grounded in ignorance of teaching (but maybe OP means that teaching math is where you can fake it most easily)