Some odometers have common problems. For several years Toyota's digital odometers had a bug that caused them to stop counting at 299,999 miles. Porsche's mechanical odometers from the late 1970's through the mid 1980's had a plastic gear that tended to strip out and cause them to stop counting at some random point.
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.