Flightstats use Sabre as their source data. (note, this excludes all AA flights). There is also an issue that for several airlines their classification of 'coach' vs 'business' cabins is not correct, hence they sometimes show zero coach when there is availability and vice versa.
idDeals.com does a better job of forecasting actual loads and takes the hassle out of trying to decipher or guess. It does require a membership fee, but worth it especially for the hotel discounts