I'm a Calhoun, however in 3 Census's my 3rd GGF last name changed it's spelling 3 time first as Cahoon, as the name is pronounced in Scotland, to Calhoon and finally as it is today Calhoun. Often times the census taker was a person who could read and write. In PA for example the German census taker might write down Johannes for John.
There is currently a DNA genealogical study going on for the surname Calhoun. This will be for the Ychromosome. I have just sent for my kit.