In bookshop I saw two books:
Help your kids with maths – An Australian step-by-step guide
Help your kids with math – Revised edition
There is no particular reason why British, Australian and New Zealand English speakers say and write maths and USA and Canadian English speakers say and write math. We just do.
Or maybe it’s not that simple. Google Ngrams shows that since 2010 math is more common that maths in BrEng, though it doesn’t show how much of that is mentions rather than uses (that is, talking about the word rather than actually using it). Conversely, maths is very rare in AmEng.
Surprisingly, the two abbreviations have been widely used only since the 1950s. Further, in AmEng, math has now overtaken mathematics.
(Compare mathematics and the almost non-existent mathematic (either as a noun or adjective).)