I was not talking so much about variations (which i get a lot with my surname even today :-) ), but more completely different name. In my case i have Mercer's, Corbins as well as my Sellens (and it variations). Now whilst i don't rule out any NPE's, we all seem to have good paper work trails backedup with atDNA.
Supposedly both Mercer and Sellens, have suggest the person was a salesman, so they both could be 'occupational surnames' in origin.
Richard