And if you aren't committing fraud and there's a reasonable reason to do this, why not do it just before sale? Why drive another 100k on them afterwards?
It's just so dumb and obvious.
It is possible FedEx's odometers all came from the same supplier and all had a MTBF of around 300,000 miles - which would explain driving another 100k miles before selling the trucks when they reach 400,000 miles.