Auto-tag scan always seems to start fresh

When I start an auto-tag scan, searching in all albums recursively, but 
with scan mode set to Non-Assigned Only and auto-tagging mode set to 
Update, the scan seems to behave as if the scan mode is set to All and 
the auto-tagging mode is set to Replace Existing. In other words, it 
seems to take just as long no matter the setting. I would assume that 
with Non-Assigned Only and Update the scan would zip through the files 
it has already processed.