The way Bypass always works is to assume that unless you make a special effort, you always hit the armor (or the heaviest armor, in this case). This is to cut down on the amount of die-rolls and steps needed to resolve combat.
If Codex was a computer game, there would still be a random chance of bypassing the armor regardless, and we would want to delve into hit locations and so on. But this would be far too many die rolls in a tabletop RPG.
By making player choice the determining factor, we allow for a bit more nuance in the combat as an OPTION, without bogging it down too much by default.