Did you read the post? I'll repost the key part:
Once you start playing a franchise, that is when the fantasy world starts. EA then basically turns into an omniscient being with the premade drafts. They know all of the Tom Bradys and Ryan Leafs before the drafts even happen. That is why they can give potential grades to these players. If this was not the case then every single rookie could turn into an overall 99 and that completely unbalances the game and is completely unreal.
Your right, they're not going to get everything right. But it's unreasonable for you to expect them to. In any case, PBP doesn' make the Brady situation any better. Brady as a rookie, would have ~58 OVR and F potential. He would never see the field.