Different in ranking between singular and plural keywords

[Die Hard]

Singular keywords and plural keywords, both ranks differently on search engines. I think, Google consider two different keywords for singular and plural keywords.  Which keyword (singular and plural keyword) is good regarding SEO ranking? And, does it matter which one you target?

[BUM]

I don't think it helps the ranking. If you use singular keyword or plural keyword, both considered as the single keyword so how can it help to increase the ranking?

[Die Hard]

I don't think it helps the ranking. If you use singular keyword or plural keyword, both considered as the single keyword so how can it help to increase the ranking?

But both plural and singular keywords ranks differently on search engines. Why?

[zerocool7]

Google does rank singular and plural keywords differently just try to target the ones that are most used. Check the Google Adwords keyword tool.