Why does a search for "spell book pack" (without the quotes) not find the spell book pack which has right the set term as title?

http://opengameart.org/content/spell-book-pack-0

I knew the art existed, but had lost the link, and wanted to find it again. It didn't show up till I only used "book" as search term and browsed the results.

I wonder how to search efficiently if I look for art? I mean in the case when I don't knwo that the art exists, I would have given up on the first search which didn't find the pack.

E.g. if I look for "spell book", which seems perfectly reasonable for what I am looking for, it doesn't find the pack at all:

http://opengameart.org/art-search?keys=spell+book

In this case I'd just assume the site just has no spell book art in the collection.