Norton will skip any unchanged files that were found to be clean in an earlier scan. The skipped files will only be rescanned if they have been modified or if Norton has received a new set of virus definitions since the prior scan. So, if your scans are within a few hours of each other, the second scan can certainly be much shorter than the first. If you look at "More Details" for one of your scans in Norton History, you can expand the Total Items Scanned listing to show how many files were skipped or trusted, and were therefore not included in the scan's file count.
I used to have that problem. It usually occurred if I used my computer during the full system scan. However, ever since I got the new version (188.8.131.52), that hasn't happened. I kept the full scan configuration that was the default and I've noticed a check mark is next to Only at Idle Time. This could be the reason as if I interrupt the scan by using my computer to go on the internet the full scan stops, but then continues the scan when the computer is idle. Go to scan history (for your last Full System Scan) and click on the underlined Full System Scan ( printed in the color blue) and see if Only at Idle time is checked. Also after my full system scan, has started ( I usually wait a little while.) if I click on do a Full System Scan, after a minute or two I will see the scan will start where it left off, if I've been using the computer ( using the internet).
I have mine set to run every 7 days (only during Idle). Mine does exactly what Holly said - start, stop when it comes out of idle and then restarts at the point where it left off after it goes back into idle.
This will cause the number of files to vary as it will only report the files in the final run of the scan. If I scanned 200,000 files the first scan, came out of idle and then finished the scan the next idle with 150,000 files - it would only show 150,000 files as scanned and the time it took to complete that section of the scan. My 7 day count starts when the final scan is run - which may be a day or two after the 7 day point passed.