Don’t forget that we only see our own shift. If a ship is out on patrol for any length of time, it would make sense to have three shifts and three crews, both on the bridge and on the lower decks, so that there is round the clock coverage. And if officers share quarters, it might make sense to share with someone who is on a different shift: you meet at change-over time, and go on or off shift, meaning the cabin always has someone in it, but doesn’t necessarily feel crowded.