Why do jurists adhere to not following the imam in an extra rak'ah, while the noble hadiths, such as "The imam is only appointed to be followed" and "Do not differ from your imam," establish the obligation to follow the imam and not differ from him, even in error?

It has been established by text and scholarly consensus that there is no addition or reduction in the prescribed prayers. If an Imam adds an extra rak'ah (unit of prayer) due to forgetfulness and does not realize it, then the ma'mum (follower) who is aware of this addition should not follow him. Instead, he should sit and wait for the Imam to say the taslim (salutation of peace). As for those who are unaware that it is an extra rak'ah, they should follow him. If the Imam is alerted but does not return [to the correct number of rak'ahs], then if he believes that what he did was correct, his prayer is valid, as is the prayer of those who separated from him. However, if he insists on the error despite knowing it, then this addition invalidates his prayer.