That's Setzer's fault for not staying in one catagory.
Are you sure he's not in 'easy listening' next to Dean Martin?
Of course there's no 'Rockabilly' catogory....it's not popular enough.
However, Elvis was always found in the Rock n Roll section in stores - even though his music also consisted of country, easy listening, pop, rockabilly. So if Brian ever had 10 number one hits....i'm sure they would move him into any cat he damn well liked!
I guess that's how it works....