eBay will provide results that have your search keywords in the item title or item-specifics (typically shown in the box at the top of the item description).
eBay will sometimes provide results with alternate keywords that are considered to be synonyms of your chosen keywords. If a slight variation of your keyword produces more results (as in the case of a spelling mistake) eBay may provide results matching the alternate word.
In some cases, keywords can be used to direct the search to a particular category -- then that keyword is ignored, and any listings in that category that match your other keywords will be returned.
If there are few or no results for a given search, eBay will provide results matching fewer words after a disclaimer.
If you are sure you are using the correct search keyword or keywords and you do not want to see any substitutes or near matches, try putting one of your search keywords in quotes (like "this"); that should prevent automatic category navigation or keyword substitutions.
