American words are spelt (spelled) phonetically, as they are sound. British spelling is closest to the Middle English form of the word, where as the American spelling is closest to its Latin ancestor.
The spelling of ‘Mum’ and ‘Mummy’
Mom and Mommy are old-English words, words that are stilled used in Birmingham and most parts of the West Midlands. It is said that when people from the West Midlands went to America many years ago they took the spelling with them, hence Americans use Mom and Mommy.
In the West Midlands the words Mum and Mummy are frowned upon as they look and sound wrong, local schools teach the spelling of Mom and Mommy and the kids still come home with handmade cards with Mom and Mommy Spelling on.
In the North East (in Tyne and Wear) they say and write Mam and Mammy.