Interestingly, when I run a manual full system scan logged on via my limited rights user account, manula full system scans always stop at around 217,000 files scanned
The explanation for this is that a user with limited rights does not have access to all the files on the system, such as an administrator would have. When you run a Norton scan with limited privileges it likewise does not have access to all of the files.
From the Norton Help Center:
Full System Scan performs a deep scan of the system to remove viruses and other security threats. It checks all boot records, files, and running processes to which the user has access. Consequently, when you run a Full System Scan with administrator privileges, it scans more files than when you run it without administrator privileges.