It is certainly strange - what build number does it show for the installation ?
The stub installer is a somewhat different case to the full off-line installation file, the stub installer would go and grab the latest avast version and start installing it. So in theory it should end up with 2015.10.2.2215 installed.

I would never use the on-line installation file, if you had another AV installed you should uninstall it or you could have conflict issues. If you did uninstall it, then effectively you would be going on-line without protection whilst you install avast.