I thought that google looked at singular and plural words the same. Is this not true? I have a few sites and I rank #1 for the term rental on them and #2 for the term rentals. If you seach with the term rental, rentals shows up bold in google in the title and description so it appears it is understanding them as the same, but why are the rankings different?
Google gives more importance for the exact term your search. If you search singular, then pages optimized with the singular form are considered as more important. I also notice the difference, but many times the ranking is slightly different.
I think you should target primarily the singular keywords and include some of the plural keywords phrases that are very close to the primary keywords.
Google ranks them separately. Exact matches will always carry more weight in their algorithm. Even though occurrences of the plural form will help you rank for the singular formand visa versa, it will never help as much as having the exact matched form in your <title>, <h1>, <h2>s, content, inbound link text, etc. As far as which to target, it totally depends on your keywords. I disagree with the previous poster who says primarily target the singular. I have some keywords that get 50x more search volume for the plural, the plural is half as competitive, and the plural converts at a much higher rate. You have to look at it on a keyword-by-keyword basis to determine which is better to target. For me the decision is based on search volume for each, competitiveness of each, and the conversion rates of each.
I am sure they have a different algos for their search results. Google I guess treats theses kinds of issues differently. More often than not, they give more weight to the exact search term.