Review miningIndexation
Apple recording your app in the search index for a word. The precondition of ranking.
Before you can rank for a word you have to be indexed for it. Indexation comes from the title, the subtitle and the keyword field; in-app purchase names and the developer name contribute too.
Apple combines words across fields: with “habit” in the title and “tracker” in the keyword field you are already indexed for “habit tracker”. Which is why writing the same word into two fields is wasted characters.