USNA Admissions has said that they work closely with MOC offices to do their best to ensure each MOC has at least one slot open each year and I'm sure the others do as well. However, they can only lead the proverbial horse to the proverbial water . . .
Let's say MOC X has the following expected vacancies (vacancies that will occur for the class year that is applying for noms):
2015 -- 1
2016 -- 2
2017 -- 1
2018 -- 1
Let's say that, at the end of the summer, the 2015 person quits USNA. That leaves the MOC with an "extra" nominating slot for 2016. Thus, instead of nominating 2, the MOC could nominate 3 (the extra filling the MOC's slot of the person who quit).
However, if the MOC uses that open slot in 2016, here's what would happen:
2016 -- 3 (the two the MOC already had for this year plus one from 2015)
2017 -- 1
2018 -- 1
2019 -- OOPS!
Assuming all of the mids stay through their 4 years, this MOC now will have no openings for the Class of 2019. Thus, USNA would encourage that MOC to "hold" that 2015 nom (for the person who quit) until 2019, so that there would be at least one opening that year. As noted, they (and the other SAs) can't force the MOC to do anything but most want to ensure their constituents have an opportunity for each SA each year.
Hope this helps.