Why is "an'anah" (reporting "on the authority of") in the Two Sahihs understood to mean connection (of transmission), while in other works it indicates a lack of connection?

Al-Hafiz Ibn Hajar, in "An-Nukat ‘ala Kitab Ibn As-Salah," discusses the statement of Ibn As-Salah and An-Nawawi that the hadiths in the Two Sahihs (Bukhari and Muslim) are to be understood as connected, meaning that their hearing is confirmed from another source, while also mentioning the objection of some scholars. Ibn Hajar specifies that the disagreement lies in what the two Shaykhs (Bukhari and Muslim) presented as evidence, whereas what came in the form of supporting narrations (mutaba‘at), there is room for leniency in their تخريج (narration/documentation).