Full system scan?

After avoiding Norton products for years because of the use of reasources and bloat I'm giving it another try using Norton Internet Security.  I was initially impressed about how quickly the scans completed until I realized that only about half of the files that other programs had scanned were being scanned by Norton.  approximately 1,200,000 files vs. 620,000.  What's up with that?  Is the scan not really scanning the entire drive?