I'm running a CRC check (.sfv) on a set of files and some of the files will fail. I'll rerun the check and some of the failed files will pass, but different files from the set that passed, will now fail. Running it a third time will give different results. How can the hash on a file change and then change back? I've tried checking w/ two different apps and with my antivirus turned on and off. What is up?