Not a bug, I have this too, and I think it's been mentioned before. It's possibly because there is still an Avast5 folder in the Alwil software program files. Maybe if I had uninstalled 5 before installing 6 it wouldn't be there, maybe not, but it's not a worry.
Probably an "Alwil", or "Alwil Software" registry key left over. With the amount of changes that go into a major version jump in antivirus software I'd personally never install say a version 7 over a pre-existing version 6, 5, 4, etc., as too much is going to change, etc., and crude up the system with left-overs or possibly cause some sort of conflict.