That's the whole point of PCS runs (4 times a day). It gives everyone a chance to check every few hours instead of having to keep an eagle eye on the open board. This way senior pilots don't miss a trip or swap just because they weren't watching.

Other airlines have a live system that is basically first come-first serve. PCS is always in seniority order for each given run.

I THINK it also runs two loops through the open time each run also, to award a WS (for example) that became available late in the first loop. This was a senior pilot has a chance to grab a trip that a junior guy swapped out of, all within the same PCS run. I could b wrong about this last part. I'm vaguely remembering someone explaining that to me a long time ago.