It is not always an even playing field, and the most conspicuous example of this is that every single congressional district has a different candidate pool, and thus the competition is different. If you are stuck in a district that has one or several incredibly qualified superstar candidates that particular year, then you better be the best of the bunch, or you chances of winning that district are low. Some districts and some states are way more competitive than others. If you are from a district in the middle of Montana, there are only three candidates, and you are the only one of the group who meets the standards to be fully qualified, guess what? You are in like Flynn, even if you have a 27 ACT and a GPA of 3.1, without varsity sports, or much in terms of leadership. On the other hand, if you have a 4.5 GPA, and a 35 ACT, Varsity football captain, Pres of student council, and a perfect CFA, you may be out, if someone else in your district is even better. And each of the academies has examples of both scenarios every year apparently.
There is also a subjective analysis which goes on, and many question the results of the same. Either way, it is what it is, and you can only do the best to put yourself in the best position for an appointment.