I cannot speak for your family but in my own research name changes happen fluidly. As families grow into new areas they change the name to differentiate themselves. Another reason is assimilation, Norwegian names for example are unbelievably hard to trace sometimes as they change dramatically over their lives. My 4G-Grandmother was horn Guri Thorkelsdatter and died Julia Anderson. My own last name goes from Barthel, to Bartl, to Bartel from 1780's to 1907.
In your case I would wager it was the differentiating factor especially with a very common name.