I believe it is simply an issue where in that the avast location doesn't change and ccleaner simply can't tell what avast version you have after avast 8.
In the same way avast 6, 7 & 8 having once detected avast 6, 7 & 8 it just doesn't clear the old entries (in ccleaner applications list) when the latest version is detected.
I haven't a clue exactly what ccleaner is looking for to identify what avast version you are using, but clearly it isn't getting it right. As it hasn't got an entry for avast 9 or avast 10.
I have done a number of clean installs of avast and it hasn't made a blind bit of difference in ccleaner, it can't really identify what version of avast is installed, NOW.
I wouldn't let ccleaner touch any avast version check boxes that it may have.