Google uses word stemming. Word stemming allows all forms of the word – singular, plural, verb form as well as similar words and synonyms to be returned for a given search query. This can work both for and against a site depending on which form of a word a page is primarily optimized for. So if someone types in "house plans", not only will pages that are optimized for that phrase be returned, but so will pages that contain all variations of that phrase, for example:
Conversely, a page that optimized for “house plans” will also be returned whenever as searcher types in any variation of that phrase. Using the same example, typing in any of the phrases below would also return a page optimized for “house plans”:
Word stemming is a helpful feature for searchers, since it saves them from having to think of many variations of a word. Word stemming can help as well as hurt your ranking for a given page as not only does it increases the number of words that you can rank well for (even if you do not include a given form of the word anywhere on a page) but it can also increase the amount of sites (competition) returned for a given search query.
When you enter a search query in Google, put a plus “+” sign in front of the word for which you want to disable stemming for. For example:
Would disable stemming on “plans” and thus only return pages for “house plans” and pages that contain variations on the “plan” word. Pay attention to stemming for your keywords – particularly to what the “root” word is and what Google considers to be a match for that word when optimizing pages over time.