I will try and explain this to you so you can understand it. It is a lot more complex than people just watching the game will understand. I have played O-line for about 8 years now. I played 5 years of college (red shirted one year of course) and 3 years of semi-pro football.
The QB calls the play in the huddle. Everyone comes to the line of scrimmage and they see how the defense is going to line up. Brady will make his adjustment calls as to who he wants blocked (for ex. you will see him pointing at who he is calling the "mike" LB so the O-line will have to make sure he is blocked) and which way the RB will block.
Dan Koppen is in charge of calling out how the line will block those players. There are calls for Max protection where the whole O-line steps down and the RB's pick up the outside rushers, You can man up and just take the man in front of you, You can slide everyone to block to their right or everyone block to their left, You can have one side of the line blocking to the right and the other side man up. Basically there are a ton of combinations of ways to block and the C has to make the call to best make sure everyone is accounted for.
This is where it gets tricky though is you have predetermined blocking schemes for certain plays. You also have an audible color or name that changes the play completely which may also change the blocking scheme. So if Koppen calls out the blocking scheme and Brady changes the play to a play that they block a certain way sometimes it can get confusing. That is why they say O-lineman have to be some of the smartest players on the team because you are always adjusting on the fly.
A quick example for you is like I said I play O-line for a semi-pro team and we had a game last saturday. We called a play in the huddle and our C called our blocking scheme at the line which was to man up and block the guy in front of you. If you don't have anyone in front of you, you look for a blitzing LB or a stunt that will bring a D-lineman right to you. He changed the play to a QB draw out of the Shotgun kind of like this Brady situation. We ALWAYS block down towards the center on the QB draw and let the DE's fly up the field because they will usually take themselves out of the play. He "forgot" that he audibled to the QB draw and set up like he was throwing a pass and got drilled from the DE off of my side. The QB and the coach started ripping my butt about it and then when the offense got together on the sidelines everyone told the coach he audibled to a draw and I was correct.
I don't think that Brady may "forget" but he has a million things going through his mind in a short period of time and breakdowns in protection sometimes will happen. I hope I was able to break it down for you a little further and get you to understand it better. Like I said there are A LOT more complexities to football than a lot of fans will understand without actually playing it. It is just like a chess game.
