Hmmmm .. I'm not sure but i have an idea, maybe the Resistance? xD .. Because i noticed, the Lightning Main is the usual class that burns fast, and those firemains who uses Mirroring Illusion users(which uses Res for the Shield) burns by chance only .. My main also burns by chance only since i have +65% Res mood and high level Res Magatamas .. Tho maybe i'm wrong, i wanna know too since i'm using Fire Main most of the time with that Standard Attack :D ..