The amount of variables from one shoot to the next would be my guess. Large gatherings vs. small, the amount of shots ordered, different filters and lenses for different shots, short videos within the stills, the time spent, etc...... The list could go on, but the point is a seasoned photographer will have price tags already set up on all of these variables so he or she can "quantify the cost faster and accurately.
A lot goes into it, that's for sure.