[bdcravens]

It's touched on here:

> Unlike with Ground, which will send a driver out for the day to do pickups/deliveries, Express drivers typically have to work around time-committed packages, meeting one or a few loop deadlines for the day, doing on-call pickups, and making a certain number of required delivery attempts.

Various Express services have very hard deadlines (For example, Fedex 2nd Day AM is 10:30AM), whereas Ground/Home Delivery can be delivered at any time during it's commitment date and still be on-time. If a package is late by even a minute then the shipper is entitled to a full refund (with exceptions for things like weather), so the Express side doesn't want Ground slowing it down (plus they were two different organizations at one point, and are still pretty siloed).

(I'm the lead developer for Refund Retriever, and our primary line of business is auditing Fedex/UPS for those late refunds)