Having different bucket availability based on individual flights vs. married flights is just a tool RM uses to try to optimize revenue (they may not always get it right, but that's what it's there for). Remember, the fares being quoted here are single through fares that covers all segments (not hop-by-hop). The $168 DL fare is a single SEA-DEN U fare that covers both SEA-SLC and SLC-DEN segments. You could book SEA-SLC and SLC-DEN flights on separate tickets with each flight on a separate fare, but it will no longer be $168 when booked as separate fares. In this case, it will be $139 + $74 = $213. This is still cheaper than the $259 through K fare, but not as low as the $168 through U fare.