There may be isolated cases, but I don't think DL is consistently that high. I recently bought a ticket for DTW-LGA for $221 all-in in October. I'm still seeing that fare as available. Even the flights that don't involve a Sat night stay are reasonable on that route on DL ($241 all-in). As for GRR, they notoriously have some of the highest fares in the country ( just look at the DOT statistics and you will see they come in as one of the higher priced airports).
Yes, I had a recent example this month where I needed to go CMH - MSP. I watched the fares for weeks. Delta was $805 for a direct flight, $605 for 2 segments each way, and Southwest was $296, including fees for early boarding. I'd like to stay loyal to Delta but no one can justify that kind of price difference!