It's most likely something to do with fare inventory. I was looking at MIA-SBN on Delta back in March and it wanted to send me MIA-ATL-MSP-SBN instead of just MIA-ATL-SBN and it was saying only First was available for $1800 one way. Ended up flying out of FLL for way less. Have also had stuff show up using miles instead of cash too which is probably a similar deal. Booked a BOS-ATL-JAX I might have to take with miles as the paid options had me sitting in ATL for 3 hours and getting to JAX at 1AM for the BOS-ATL I wanted while an option landing an hour and some change earlier into JAX was available using miles so I did that. Might be worth seeing if anything new comes up with miles just to see.