Where are FedEx's expenses?

I shipped four boxes to San Francisco from Chicago on Friday. They were shipped "FedEx Express Saver", their three-day service, which means they should be there by Wednesday, the 12th of March.

The boxes actually got to SFO on Saturday the 8th. They sat around there until the 11th, when they were moved to South San Francisco, which is where the stuff actually gets put on trucks for final delivery. The South San Francisco station looked at the packages, said "Hey, don't need to deliver this until tomorrow" and put them on a shelf.

I wonder where FedEx's costs are. Warehouse space is expensive, especially in the Bay Area. Do they really save more money by hanging on to my packages, tying up space, risking damage and loss, etc., rather than getting rid of them? I can see it making sense for FedEx to hold them in Chicago, if adding my packages to the SF flight would mean that they would need an additional plane that day. But holding them once they get there seems odd.

For the record, this is not a complaint about FedEx -- I am getting the service that I paid for, I'm just curious.