In their indexes, Zettelkasten practitioners try to tag only the most important few notes for a given topic, then they rely on inter-note linkages to navigate from there (see Indexed references vs. tags). This keeps the entry high-signal (see Tags are an ineffective association structure).
But this may just be a consequence of current interfaces for displaying tagged data. Conceptually, what’s happening here is that in the writer’s mind, many notes are tagged with a given tag, but some notes are particularly relevant to that tag, and the writer doesn’t want to lose track of those. So they create a curated representation of that tag to replace the system’s indifferent “list it all” visualization.
But if you could view a tag’s network structure, arranged by PageRank or something, I suspect this would be come much less important.
Related: Are literature notes necessary if we have automatic universal backlinks?