I work for DHL, so I know of what you speak, so to speak.
I can't explain the McAllen to Houston trek, but the main hub for ALL major carriers (DHL, UPS, FedEx) is in Kentucky. Well, DHL's is in Willmington OH but it's across the border). Every package is flown from it's original departure point to the main hub where the plane is emptied, and package are sorted according to final destination.
So your package went on a plane with all the other packages coming from the McAllen sub-station, flew into Kentucky (via Houston for some reason) then got sorted and put on a plane that will go into the airport in LA where UPS will then distribute them into trucks going to all the UPS sub-stations in LA for final delivery.
Yeah, it's re ed.