I have always found Mom weird.
I have always found Mom weird.
If you can believe Wikipedia - https://en.wikipedia.org/wiki/American_and_British_English_s... - Mom is sporadically regionally found in the UK (e.g., in West Midlands English). Some British and Irish dialects have mam,[117] and this is often used in Northern English, Hiberno-English, and Welsh English. Scottish English may also use mam, ma, or maw. In the American region of New England, especially in the case of the Boston accent, the British pronunciation of mum is often retained, while it is still spelled mom. In Canada, there are both mom and mum; Canadians often say mum and write mom.[118] In Australia and New Zealand, mum is used. In the sense of a preserved corpse, mummy is always used.
It really has more elements that sound like West Country accent, Irish English, or even Scots/Ulster Scots.
English and American dialects both originated in (Shakespearean) Early Modern English, but both have changed over time, as languages do.
Now what's interesting is that this sound is closer to how an American might pronounce a short "o" (I think of a southern belle fanning herself and saying "lawd it's haaaht") and how an Englishman might say a short "u". It's not exactly the same vowel but I can see how it develops.
Both versions of English think babies say "mama", but you can't shorten that to "mam" because you'd use a different "a" to pronounce that.