Full Scans vary in the number of files they scan

For some reason my full system scans vary sometimes, usually it will scan about 558,000 files but sometimes it will only scan about 200,000 files. I dont tweak anything or mess with any of the sttings, I just click the button and let it scan. Can someone tell me why this is happening? Also do you think 59 minutes to scan 558,000 files is slow?