I'm not, try seeing this from the managers perspective for a second.
You have a limited budget, FFP is making things tight. You don't trust your GK 100% but he's done okay for most of the season, even though he's causing you to compromise on the football you want to play.
You agree to renew him and keep him on for one more year as first choice so you start negotiations with the player. He then signs a deal on reduced terms to be the number 1 next season and goes on to cost you multiple big games in the season, including a final, at which point you realise that this guy could actually cost you your job if you finish trophiless at a big club like United.
So you change the terms of the deal instead and agree to keep him on but not as first choice, based on HIS performances.
Contracts during Fergie's time were almost always done in the offseason, there's no value in giving new contracts in the middle of the season for this exact reason, you end up having to change the terms if the player isn't good enough or your transfer plans change.