If the outcome of a battle of jutsu was just determined by the elemental affinities there wouldn't really be a point in fighting.
The people would just tell each other what element they have and that would decide the winner *gg*.
They elemental affinities are one part of the equation that decides the outcome. The strenght of the jutsu is another part (and a more important I might add).
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"