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Recall the importance of the page title in establishing page relevance. The page title in this instance is using the plural version of your targeted keyword. Perhaps this is factor alone is influencing the ranking priorities of the search bots and producing the results you are seeing.

I do see the singular version of the keyword appears in several instances on the page, but is wrapped in an

with an

appearing _prior - _which struck me as a bit odd.

Sorry, but I didn't have time to dig into linking profiles

From Google's perspective on the matter, both versions of the url are treated equally; therefore, I can't surmise why they wouldn't pass the same amount or link juice.

An excellent discussion regarding the the two versions of the url may be treated differently by redirects or how they may effect user experience can be found on the Webmaster Central Blog:

http://googlewebmastercentral.blogspot.com/2010/04/to-slash-or-not-to-slash.html

Hope this helps.