I have a similar situation to my great grandmother's Sizer family, Cambs. (Soham) and the Alford area of Lincs. I knew two Coulson's at Boston Lincs. 50 years ago, but their antecedents were quite likely in Cambs I would have thought. I believe that it is likely that where there are 2 widely separated groups with the same surname then one of them may have been moved by a land owner to work on another of his estates many miles away.