I did have a weird thought, in a organisation as big as FedEx, would they have a pool of spare engines to minimise downtime? You could get unlucky and get a can with an engine that's been through a few chassis and end up with an ECU that knows the engine has done 400k miles whilst the chassis has only done 180k?
Although that's probably a mad theory, if there is hijinks it probably is the good old fashioned kind, probably from this 3rd party middleman rather than FedEx themselves though.