SEP 12.1.2 full scan results inconsistent

We're testing SEP 12.1.2 on a 64-bit Windows 7 Pro client.    When we run a full scan, sometimes we get a large difference in the number of files scanned.   For example, sometimes SEP will report 170,000 files scanned, then if we immediately run another full scan, SEP will report 80,000 files scanned.  

Has anyone come across this?  Could be something to do with new AV defs being loaded and SEP scanning.    We are aware that the scans have been improved over SEP 11 and that SEP 12.1 keeps track of what has already been scanned so it won't have to scan them again.   Could be something like SEP rescanning everything on the first manual full scan after new AV defs are installed.

We appreciate any enlightenment anyone can provide.