From my understanding, the search engine searches for the "tags" that the artists make on the items. When the files are named they are named like this: Knight_Human[Knight,soldier,token,human,etc] The first part of the name before the brackets is what is shown in your library. The words inside the brackets are the tags that will result in the search function. The search results also place the most "popular" items first, and that unfortunately means the older sets that have gained more clicks due to being out for years and years. I by no means am making an official statement or answer to this, just an observation through the years of searching and creating art myself. It's great for the artist that have been around a long time and creates a perpetual popularity for their search results. I'm pretty sure there is a thread on the suggestions forum that has a good bit of traction and upvotes for fixing the tagging system. I've also noticed a lot of artists use the same tags on an entire set so their actual file names will tag the entire set into a search for one item, thus your search for "knight" will often result in random tokens coming up like a barbarian, an archer, and a mage, because they are tagged the same way because they are in the same set with a token that has a knight in it. They are also tagged as "barbarian, mage, monk, archer, etc".
