When you are seeking to exploit the maximum from your Zettelkasten system of knowledge management you need to understand that understanding is more important than learning.
Remembering everything you hear is of no benefit to you if you cannot make connections between relevant, meaningful ideas and concepts. These connections allow you to retrieve the information you need when you need it.
Remembering everything will only flood your mind and brain with a lot of details, some of which are relevant to a given idea, some of which are of no relevance. The key thing is the connection between new notes and old note, between what you have now learned and previous information and knowledge.
The slip box system allows us to make theses connections without a hierarchy or division of organisation into folders. It is a bottom-up system, not top down and not based on folders.
One of the advantages of this system is to allow you to make connections between apparently disparate ideas and concepts. Too much order can prevent these connections being made. For this reason, you need to constantly review your notes to ensure you do not forget old notes and to compare and review them with newly acquired information. You may also need to tweak previously made notes in the light of new knowledge, facts, data.