In general it could be any of a few things:
- At the edge of the schedule not all the discount fares are published
- At the edge of the schedule they don't release any of the inventory lower than full fare
- The new rev mgmt system (gemini) generally seems to be increasing extreme advance purchase fares
- They're aware of the scheduled event and expected demand, and holding out
It's not the first one, the first class fares are published all the way down to KxxxxFN at $360++.
It is the second one, the 4/22 flights are all J9 C0 D0 Z0 P0 Y9 B9 M0 E0 U0 H0 Q0 V0 W0 S0 T0 L0 K0 G0 N0
It's likely not the third one, since we haven't otherwise seen it this aggressive
It's very likely not the fourth one, since they don't really plan that much for special events