"Maths" is short for "mathematics", "math" must then be short for "mathematic".
Just like plenty of other short versions of words, if it's a plural
Hey, I'm going to mathematic, I'll see you after class.
No no no no, no no no.
Pants? Pantaloons... trousers - now that makes more sense than the English word for underwear, but "math" is plain wrong

Also Selby, we have all of the above too, it usually all just gets thrown under the heading of maths class - instead of algebra class etc