Arithmancy had enough mathematical concepts for him to enjoy teaching it even if it wasn't his field of expertise. He was still skilled above and beyond all of the people who had applied for the position. He was intelligent and he taught well, so he had no problems during the year.
"It does. I'm quite intelligent and Emrys wanted the best." Which meant he wanted Tony in this case. There wasn't anyone better for the class than he was, and if he wanted a small accommodation the headmaster was willing to make it to keep him happy - especially since he willingly took a lower pay scale - not that he would tell that to anyone.