With further searching, I found this very same family with the name Hedderman in the 1900 and Heiderman in the 1880 were once Hetherman's, in the 1860's and 1870's I am guessing that either the Census taker wrote their name as it was pronounced (Irish Accent) or at that time a lot immigrants from western Ireland were illiterate and didn't know themselves, on how to spell their names. I know that the further away from England you lived during that time, the more illiterate one was. So the spelling has probably changed over the years due to their accents and their illiteracy. But again.... this is only my guess. Good luck in your family search!