mysite.com and mysiteS.com problem

2 mirror sites, one with ton links back, 2nd with none, but 2nd ranked high

[mnemonic30]

I have very unusual problem (sort of problem which would say link relevancy aint important at all)

On a quite website online for 5 years (over 50k visitors a day, primary bookmarks, site probably never got to many hits from google in all those years) situation is as follows:
-there is primary domain www.mysites.com (dont overlook 2 S in domainname) which is better knows, and there is www.mysite.com (without S at the end) which is less known. Probably only reason when people visits www.mysite.com or mysite.com is when they made a typo error when typing in an address in address bar
-mysites have good link relevancy (linked back by many websites, over 100)
-mysite have NONE link relevancy (at least by google info)
-sites are identical (mirrors)

however against all common logic mysite.com has a lot better ranking (both are indexed, on each more then 90% pages indexed, out of 50 000 pages)

Anyone with similar problem? Any idea why this happens?