I don't have much to add compared to
JAXPax 's fascinating answer, but I've heard that DL wrote it into the handling contracts that the 2500 miles that you get for the 20 minute baggage handling guarantee is paid for by the below-the-wing contractor.
Interesting! Now that I think about it, I seem to have slightly more 2500 mile bag claims from mainline DL stations, and not contracted ones. Could explain the slightly better push to get bags out on time.