The commentary contrasts a Qur’anic use of idha with al-hasanah (good fortune) and in with al-sayyi’ah (misfortune). The distinction concerns the rhetorical presentation of how the respective conditions occur.
Al-hasanah can be taken as a broad kind whose occurrence is customary, whereas al-sayyi’ah may be taken in a more particular sense whose occurrence is relatively uncommon. This is a comparison of the general kind and a particular subtype, not an unqualified empirical claim about all good and bad events.
The interpretation depends on what the utterance treats as the relevant domain.