Part of the confusion about LOA's is that each SA has its own admissions process.
For example USNA uses a "rolling admissions" selection process and divides qualified candidates into two categories, one of which leads to an LOA:
Applicants who qualify for admission are categorized by the following:
1. Outstanding Achievement - Students who have an outstanding achievement record will receive a Letter of Assurance. This letter indicates our intent to extend an Offer of Appointment and could be received as early as September of your senior year in high school.
2. Board Qualified - Students who do not receive a Letter of Assurance will be selected from within your nominating sources for an Offer of Appointment.
West Point does not use rolling admissions (stopped several years ago). Competition is open until the application deadline. Candidate files are classified using ten categories that may change during the admissions process.
None of of these categories leads to an automatic LOA. Why? Because if an RC gives a candidate an LOA and a more qualified candidate later comes along in the same district before the application deadline, USMA then has to find an appointment for the LOA holder - a logistical mess when applied across hundreds of districts.
USMA only offers LOA's to candidates certain to win an appointment such as: win a slate or be appointed as a Qualified Alternate (hard to project early in process), or be appointed as an Additional Appointee (easy to project - primarily recruited athletes and diversity candidates). For simplicity I left out other appointment categories, since they don't apply to most candidates.