CO's business model favor those (and consequently upgrades those) who pay the highest fares. It is well known and it is designed to encourage flyers to book Y and B fares to get up front. In contrast, AA favors those who purchase first and request the upgrade, within each elite level. regardless of fare paid. The first EXP who purchased any fare for a given flight will trump another who paid a higher fare and requested the upgrade on a later date. The only exception is when one purchases a P fare, usually KUP or YUP, in which case one either automatically upgrades or goes to the top of the list. IME, there is only a limited pool of KUP and YUP fares on each AA flight and unlike CO, not all Y fares are YUP fares.

On CO any silver on a Y or B fare is going to ride in first before any plat or gold on a lower fare, assuming that the tickets were purchased before the EUA windows open. AA protects its highest level elites more than CO and if all the P fares are sold, the only way a Plat or Gold will jump the queue is by purchasing some type of a real F fare.