I guess my excuse is that English isn't my first language, so I blame my occasional slip-up between BE and AE on the ever present American English in my life th

I guess my excuse is that English isn't my first language, so I blame my occasional slip-up between BE and AE on the ever present American English in my life through 99,9 % of all the text and media I consume. I actually hadn't thought about "mum" being a British variant, but now that you mention it it's suddenly glaringly obvious.