Essentially, yes. However, the answer is a bit more complicated and depends to which nominating source the candidate was "slotted." It could be the Congressman, or the Senator, or another source. That, in part, depends on whether the nom was a principle nom or a competitive nom. At the end of the process, however, each MIDN is slotted to only one source, thus freeing up spots with the other sources. Hope that makes sense.
One other note -- it could be that a MOC has no MIDN at USNA for a given year. Suppose the MOC nominates ten candidates for one vacancy for the year. Some of those 10 may end up not fully qualified (usually medically, if anything). The others may decline offers of appointment. If that happens (no one takes the slot), that slot will be an "extra" for the MOC the following year.