Your Gpa is based on the classes you've taken and the grades, calculated against the classes available and what you took. In other words, the academy has their own weighting formula. Based on your senior schedule and your school profile, some gpa' are exactly what you calculate. Some weight higher. Some weight lower. E.g. 6 classes all AP classes with all "A" could be a 4.0 or depending on what the AP classes were, could be higher or lower. 6 classes with. All A's but none AP classes could still be a 4.0 if your school doesn't offer any AP classes or you've taken all they had. But if they had AP classes and you didn't take any/many, then all A's could be less than a 4.0.
This is one of those times where if you think your gpa is grossly lower than you think it should be, that you should call your academy admissions counselor to look at it again. It's also possibly they made a mistake inputting data. They are human. But definitely worth a call.