Lots of folks get noms and not LOAs. It's the norm. You can get an LOA and not get a nom -- I've addressed the reasons for this on other threads. It is rare to get an LOA and not get an appointment for other than medical reasons -- however, USNA Admissions told the BGOs that it has happened and I believe them.
My GUESS -- and it's only that -- is the most likely reason you could get an LOA and not an appt is that USNA moved too quickly. Sometimes, they offer an LOA to a candidate who -- based on incomplete information -- seems like exactly what they want. As more info comes in (e.g., teacher recs, CFA scores, BGO interview), there may be evidence that this person isn't as great as originally thought. The MOC nom cttees may have the same view and not provide a nom. USNA, rather than pull the LOA, may say: "no nom, no appt." Again, just a guess -- definitely NOT official policy.
Bottom line: If you're one of the few to get an LOA from USNA, don't sit on your laurels, so to speak. Don't count on the fact that USNA will "find" you a nom -- maybe they will, maybe they won't. Make every effort to secure a nom on your own.