In some cases, a general search may pull in items that are listed in the wrong category, or items that include inappropriate keywords. Sometimes sellers purposefully list their items incorrectly to wind up in more searches, or else just make mistakes. In some cases, eBay may be doing some sort of keyword substitution or keyword-to-category mapping where it is not appropriate.
Putting one keyword in quotes or using an exclusion term should avoid the search term substitutions and automatic category navigation, in most cases.
Try starting with a general search of your brand item, and then narrow to a particular category if the search does not do that already. You may find category-specific filters that allow you to narrow your search and eliminate many items that are in the wrong category. Filters for size and type can be helpful -- these often work much better than trying to use a keyword that may be misinterpreted or may be present in another item-specific field. If there are listings that have multiple variations, as long as one variation matches your search the listing will appear in your results.
If you could provide an actual example that illustrates the sort of problem you are encountering, I might be able to give more practical suggestions rather than just general hints.
