When sites.google.com first came out it ranked their own pages much better than a new site (they've since turned it down a bit). Many tech reporters say the only reason they use Google+ is to post stories on there because Google will rank it more highly than their own site.

it's really quite simple. The pages that get more +1s and likes and comments and whatever metric you want are the ones that are good. The ones that are good also get more links and other signals that are used in algorithms. The hidden middle-factor here is "quality."

That, or simply: "the pages that have more +1s have them because they ranked and got more traffic. e.g. nobody's +1ing pages that they can't find."