An LOA is essentially a promissory note, pending the resolution of some contingency. Because of that contingency, the LOA is not — contrary to many opinions — a guarantee of appointment.
The LOA could be contingent on, for example, securing a nomination, receiving a medical waiver, explaining an academic issue, retaking the SAT/ACT, completing the BGO interview, getting a better CFA score...frankly, any number of things.
An LOA is an academy’s way of telling you that you’ll receive offer of appointment if — and only if — the contingency is addressed to their satisfaction. So again, not a guarantee.
Finally, please note: LOAs are extremely rare. LOAs can come at any time, as late as the day before civilian colleges require a decision. And most importantly, LOAs are not the goal — an actual offer of appointment is.