Well, it depends on how common "most common" is, but one possibility is that there's a non-paternity back somewhere, adoption, illegitimacy, legal name change, switched at birth, etc. The likelihood is fairly low in a particular generation, but it's been around 20 generations (600ish years) since surnames were universally adopted in England, and the chances of at least one non-paternity in 20 generations get to be pretty decent.
It's also possible that the connection predates surnames. Basically, your ancestor Adam could have had a brother or a cousin or other paternal relation who was a smith. Their descendants could then end up with the same Y-DNA but different surnames.