There are many theories, but nobody but admissions knows how, why, or when they are handed out. My advice to you, and it's been written in these forums many times... Don't worry or even think about an LOA. Concentrate on the appointment. LOAs are great, but they are not an appointment. My son got an LOA, and we were excited for him. That excitement eased up after a few weeks when we realized nothing really changed. He had to wait over 6 months to get a nomination.