I'm guessing WP is using "natural search" mode. We use "boolean search" mode. We've chosen the boolean mode because otherwise (in a "natural" mode) mysql uses 50% threshold, so that most common search words produce no results - this can be quite confusing. For example, if you have a store dedicated to shoes or dogs and someone performs a search with the word "shoes" or "dogs" in the search phrase, most likely no results will be returned, even though more than half of the products have that word in them.
The downside is there are character limits for search.