Avast free antivirus on Windows 8.1. Question about the number of tested files

My PC uses Windows 8.1 and it’s for domestic home office use. I scan with avast! regularly and it works well, but I don’t understand the reports. If avast! finds a file it cannot scan it tests many more files than if I delete the unscannable file and repeat the test the next night. If I take the report literally it’s implying that there are 4,500,000 files in the zipped file that avast! cannot scan, almost the same number of files that exist on my whole PC and that seems intuitively very unlikely. So are the numbers inaccurate or what? Details: Both scans are boot scans. I scan everything, all drives and I haven’t altered the boot scan parameters between scans. The details of the scans are:-

11/14/2014 23:45
Scan of all local drives

File C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Temp\mpam-e2fff1b2.exe|>mpasbase.vdm Error 42127 {CAB archive is corrupted.}
Number of searched folders: 83407
Number of tested files: 10541260
Number of infected files: 0


11/15/2014 22:16
Scan of all local drives

Number of searched folders: 49047
Number of tested files: 5924735
Number of infected files: 0

Thanks for your help and regards,
Nigel

I’ll renew this topic because I’m still getting this effect. Recently I’ve been doing boot scans and here are a couple of results:-

08/02/2015 22:28
Scan of all local drives

Number of searched folders: 50120
Number of tested files: 4870758
Number of infected files: 0

08/08/2015 23:25
Scan of all local drives

Number of searched folders: 86284
Number of tested files: 9559556
Number of infected files: 0

The numbers vary greatly but I’ve done very little maintenance - upgraded Firefox, that sort of thing.
What’s going on please?
Regards,
Nigel

When the scan appears to check twice as many files, does it take twice as long?
Perhaps it’s just something odd in the reporting.