Archival files do not do anything to your system unless you or a program/process opens the file. Avast, when used by you to scan all files (full scan), opens all files that it can and checks the contents inside. Avast is your program or process performing the action in this case.
A manual scan (monthly scan in your case) is one that you initiate. Real-time scans are what block malware from ever running on your system. So it is the real-time scanning you can count on to protect your system from harm, and the manual scanning you do will give you peace of mind. A manual scan does not actually protect your system from harm, the real-time does.
Suggest changing your default action to quarantine first then fix automatically so you can recover a file if need be and it is found to be clean in the future.
I thought that might be the case so as why no quarantined files were found by you.
