Looking at certain routes, for example ORD-SEA 6/27, the lowest fare class available is U at $518/$484, and per ExpertFlyer, the lowest published and validated fare is K at $93+fees. This would make sense if the flight is very full but say for UA1945, the flight is only about 1/3 booked. Is UA intentionally zeroing out lower fare classes in this case? What could be UA's reasoning for this since it's close-ish to the departure and there isn't really anything happening in SEA afaik?

Picking another date that's not close to July 4th, say 7/19 same flight UA1945, it's selling at Q5 but K fare published at $93 and the flight is also only about 1/3 full.