somewhat on target with this.
It isn't an overselect as it is a manning projection, and the USN can only commission X amount of officers per year, so if they project they need 50 more they would need to do that by taking from some other designator.
An overselect means they picked more than they can ship to OCS, that results in board cancellations and such.
The main reasons you would see selections in SWO being X+25 for example when they only need X is that people get selected that have SWO listed 2 and 3, when they get their 1st choice they drop SWO.