Hi Sally
Quite honestly literally anything can happen on a census form. There were good enumerators and bad ones, and who knows what happened here. He could have written it roughly to write in properly when he got home, couldn't read his own handwriting, maybe just wrote Mt and then read it as Ht. Although there were strict rules on what they should do mistakes often happened as I'm sure others on rootschat can tell you. Plus, at a time when so many people were still illiterate everything was down to the enumerator and spelling was also a great issue.
Would think maybe the birth certificate of the youngest child would be the best bet.
Look forward to learning the result.
Annette