@zatnosk for people who have English as their first language (particularly American English) vowels don't really matter. This means that in many (or most) american accents 'mastodon' and 'mastadon' sound the same.
So that may have something to do with it. I had to check to see if the spelling was mastodon or mastadon when I saw your toot but I am particularly bad at spelling so I may not be a good example.