Yes, I am a direct Y-DNA descendant. There was most likely an adoption or an illegitimate birth. Adoptions happened all the time back then, Both parents die and the child is adopted by another family member - no paperwork or anything. The child gets a different last name than his biological father.
When I look at all of the Y-DNA close matches, they all have the last name of MacKay, McKay, or McCoy which are all variations of the same name, and they all have the same common ancestor - Robert MacKay born 1679.
I just am not sure where the adoption took place, but I have my suspicions, just no proof. I am waiting for more people to take the Y-DNA test. Until then I was wondering how to include my name on the DNA connections on Robert MacKay's profile.