Different ship dates pay different bonuses.
The bigger picture is that the bean counters need to fill quotas, to fill billets, to send ships to sea, and let Sailors take shore billets so they can come shore side and let the newbies fill their spots, get experience, and the circle of life continues....sing it Elton, sing it for Simba.
If they need you bad to ship, you get to profit on that need. If not, you don't.