Just checked Andrew Luck (for the OS community rosters) and his tendency on the menu is a Scrambler. If you go in to edit his attributes, it shows that his attributes equate to a balanced QB.
All in all, I'm thinking it's more of a bug in the game that changes the tendencies. Kind of like the reason the player faces are disproportionate for some players, in relativity to their height/weight. Ironically, that's caused by a player being a certain height, not even necessarily weight.