Thanks guys. OP here.

What you're talking about is definitely a known - and infuriating (or should I say Furiating?) - problem.

It seems pretty stupid to me, but you can buy fares on 3rd party sites that DL.com won't offer you.

I've written to DL about it and gotten the same useless stock responses you get for other problems.

I'm actually talking about something a little different.

How is it possible to have Y0 and T4...that means that if you want to walk up and pay full Y they won't sell you a ticket, but if you have an L ticket, you can pay the change fee and get onto the flight with no fare difference??

Makes no sense, and I can't believe that it's accurate.

I'm trying to get a feel for what I can rely on from flightstats.