If you clean your older installations of avast with the removal utility, and you manually delete all (hidden) folder that contain "avast" or "alwil" in their name, a new clean installation of avast will be using a new path.
But avast needs to support the update method anyway, so the old path is still valid, as it should.
The problem is no in avast, but in CCleaner. CCleaner detects the path, and according to it, then the list of items is built. That's how it works. You could have "anything" in that path, and CCleaner would be adding the same item.
If you right click on the item ("Avast 5") and select "analyze", you will see the list of logs that CCleaner detects (which might be not the complete set). If you do it in the other item ("Avast 6"), the resulting list on the right pane will be blank.
Conclusion: CCleaner detects the path, no matter which version of avast you really have.