Full Scan Virus

I  am curious as to why I sometimes have the full scan ,which I do every Sunday, take about 41 minutes to complete and other times about 1 hour and still other times just over 3 hours. Also I have anywhere from 500,000 files scanned to about 860,000 files scanned. i usually haven't added anything from one week to the next week. The scan is set for only do when Idle. I don't go on the computer when it's doing a scan. Also before the scan I always make sure I do a disk cleanup and delete unnecessary files.

 

Holly