There's a translation of cross-airline fare codes that I haven't figured out.
Before UA was nice enough to throw me some status, I used to use my DL# on UA flights. I'd fly UA in Y (actual class paid) and it would appear on my DL statement as D class.
D on Delta is discounted C (like A is to F or B is to Y). Now, rest assured, DL did not give me 125% miles credit (then the applicable bonus for C/D/I/J), nor did I get a 100% GM bonus.
A previous post talks about fares classes mixed up between NW and CO, too.
We'll see.