I glanced through the reviews that make up Fred S's score and a lot of them seemed like middle management types. Box slingers and cardboard pushers aren't going to be leaving the bulk of reviews on GlassDoor.
The couriers and loaders at FedEx certainly aren't happy - they were due a raise recently that was (permanently?) deferred.
Maybe it's because most junior management folks (the highly qualified ones with degrees that know how to network and jump ship) at UPS have been bled dry since 2008?
Edit: Scott Davis has a 66% (friend) approval rating