You are correct. Only a few get them. It's sort of like "early admissions" from a traditional college. Not to sound elitist, but usually the ones who get an LOA, are what one might call the "No Brainers Club". Meaning that they have the resume/application of such, that they pretty much would get an appointment, no matter who they were competing with. The academy has decades of experience knowing what the below, average, above, and exceptional applications are. Mind you, even with an exceptional application, that doesn't guarantee an LOA. Just a better chance. They give out about 100-150 per year.