The Rank calculation used is this page is taken from Freshmeat, for an explanation see here
weighted rank (WR) = (v ÷ (v+m)) × R + (m ÷ (v+m)) × C where: R = average for the trig (mean) = (average score) v = number of votes for the trig = (visits) m = minimum votes required to be listed in the top 10(currently 1) C = the mean vote across the whole report (currently 5.5568947271556)