To put it simply, Statcounter is a server-side stats package that tends to count everything (bots that send a request to the server included), and tends to record pages that were requested but never loaded (users clicking 'back' before the page had time to load).
More detail on this here: http://middlekingdomlife.com/guide/analysis-of-the-statistical-differences-between-statcounter-and-google-analytics.htm