I have a degree but it isn't actually what it looks like it is. How do I list it on my CV?
I would list it directly with the degree. I have a similar issue in that my degree is Geodetic Science, but my focus was in (at the time) computer-assisted mapping, now known as GIS. People try to call me a geodesist, but I'm not really. I would use something like:

M.S. Mathematics (specialty: Computer Science)

Edit: I finally remembered what the various tracks were called in our department--a concentration, not a specialty. Other synonyms include focus, even research or research topic.