Americans say math, but people from other English-speaking countries say maths. Why is that? They say it's because it's short for mathematics, which is plural.

But consider this: fish can mean multiple fishes. But you don't always have to say fishes. "Fish" can be plural on its own. I caught some fish today. You don't have to say I caught some fishes.

Countability is an issue in English, where it matters for some words, but not others. English is a mess, where rules are made and broken all the time.