There's so much to it. Example. Just using one semester. 4 kids. 4 classes each. All 4 took 11th grade math, history, science, English. All 4 kids got "A" in each class. One kid took state minimum required classes. One kid took 2 state required classes and honors English and history. One kid honors English and history, and AP math and science. Fourth kid was in the IB program and took IB math equal to trig or above, IB chem, IB world history, and IB literature.
Now, take this information, and add that the IB student and the first kid are in schools that don't weight gpa. They both have 4.0. The other 2 kids are in schools that weight, and one has a 4.5 and the other a 5.0 gpa.
Now add to this that the first kid who only took state required classes lives in a town of 200 people and their school doesn't offer honors, AP classes or the IB program. Not their fault that they don't have advanced classes. They aren't available. The 2nd kid had all the honors, AP, and IB classes available, but CHOSE to only take 1 advanced honors class. The kid taking the AP classes took as many as were available. And the fourth was fortunate enough to be in the IB program where ALL classes are advanced.
Now, figure out how to fairly give all four students a gpa based on the same scale, where they are accurately and fairly compared to each other. That's what the academy has to try and do. Do they always get it right? No. My son is a perfect example. He was the 4th kid. Did 4 years of high school in the IB program. Had a 4.0 gpa his entire life in school. Never below an "A". School didn't weight his 4.0 differently than the 4.0 kid taking minimum required classes. And the academy showed my sons gpa on the application as a 4.0. Yet, there are no more difficult classes available than the IB program in just about any school.
So, you can see how complicated this can be for the academy. And I haven't even included the kids who take additional classes at the local community college or the home schooled kid.
So what does this all mean? It means do your best with what you have to work with, and don't worry about the things you have no control over. Put your efforts into being the best you can be and not what others do.