I always post this link when these type of question come up because I think it is pretty cool website so try your name here.
http://worldnames.publicprofiler.org/Main.aspx
I actually typed it in with both spellings and it kind of affirms what you are saying. That Renfrew comes up mainly in Scotland and Renfro seems to be predominantly American. Perhaps due to a change to the spelling to the surname on arrival which was not an uncommon affair.