Just bought a ticket for my son on NWA.com. I had choice of two itineraries, both the same except the first leg: either the 5:35pm SLC-MSP or the 5:10pm SLC-MSP on Delta. My son wanted the 5:35pm flight cause a friend of his is travelling on it, but the 5:10 DL metal was $45 cheaper, even though all the other legs were identical.

Why would NWA be selling seats cheaper on a DL flight than on an NWA flight?