Why is accusing a Muslim brother of disbelief considered a minor disbelief (a point of contention among scholars), while accusing oneself of that is considered a major disbelief?

Whoever accuses his brother of kufr (disbelief) is considered to have committed minor kufr by the majority of scholars, with the exception of Abu Ishaq al-Isfara'ini. As for a person accusing himself of kufr, meaning reporting about himself that he is a disbeliever, this is something that expels him from the fold of Islam, unless he was coerced into it by a legally recognized coercion. The difference is that the first accuses another of kufr, while the second accuses himself of kufr.