Just a guess:-
It might not have install by itself but it, probably, updated from the "earlier version". If it was not the offline installer of whatever version that was, as I understand it, AVAST downloads and installs the latest version.

You might not even have noticed this as the version of AVAST that was displayed would most likely not have changed until the system rebooted.
Alternatively it was the CCleaner install that did it as now, apparently, that includes a pre-ticked AVAST installer. Unchecky should have dealt with that but there have been issues with CCleaner this year were several versions have ignored other user settings despite them being unticked. So that is a
possible explanation.