Basically a lot of global logistics runs on trust.
If a driver is delivering a pallet to the FooCorp warehouse, he doesn't get given a copy of the FooCorp org chart, or get an example signature to compare against the signature they're given, or get given a map or a secret password or anything like that.
He just pulls up to the building that says FooCorp on it, says "got a delivery for FooCorp", they let him in and he accepts any name and signature from whoever is near the door.
Obviously. But if there is 400 grand on the line, you'd think someone would actually check(when the claim is made). The receiver would say "you have a signature from person X. Person X doesn't actually work here". Fedex then says "ok, prove it" - and then the receiver does, in whatever way is legally acceptable.
Edit: in fact, let me add a bit more - if the shipment was delivered to the right address just signed by someone who didn't actually work there then sure, I think FedEx would be in the clear. But they delivered the parcel to the wrong place - the fact that it was signed for by someone is almost irrelevant, it's the same as having no signature at all.
[1] I'm not sure how they check the number, because they accept the ID number of anyone in the family and even the full time cleaner of the building. They type it and the app takes a second to show a green check. I guess it's a fuzzy list to prevent obvious scams.