It has always "appeared" they need people to stay, but that is because retention is important to the Department Head, I had one 2nd sea tour nuke officer explain it to me, the standard was each carrier had REA and MPA as SWO N these are 2nd nuke sea tour spots, and the LDO's had CRA and RMA spots, if you look at the DIVO's for SWO N it would be about 10-12 give or take, so there were 10-12 to feed into 2, now some carriers have the CRA as SWO N to give more opportunity, and some have split the REA into RCA and REA as well when needed.
One LT I knew was told by his DH that the USN needed him to stay, the detailer was doing a ship visit and the detailer told him flat out, "I don't know what your DH has told you, we don't need people to stay, we have enough, if you want to stay you take what I give you", another had orders changed, the detailer said, "don't like it, put in your letter, we have met our numbers"