The search is focussing on one thing. In your example, artist or title. Your search for a specific title will concentrate on that aspect. Your search for the artist will concentrate upon that aspect. A search has a certain amount of energy; it times out after a while. If sufficient results are found for the main aspect of the search, then you may miss out on some other results.

Also consider there might be some relevance to some not having id3 tags or the tags are different in such a way that different search criteria give different results.