How do you explain spelling Tatty with an 'a' and spelling Mommy with an 'o'?
It should either be Totty and Mommy or Tatty and Mammy...
Do you really want to know?
It's actually fairly simple: Mommy is already an English word (albeit, pronounced slightly differently) so there was no need to invent a new spelling. Tatty, on the other hand, is a transliteration of a Yiddish word, so it was spelled in the way that is closer (although not exact) to the correct pronunciation.