saddle reference point
Handlebar drop by default is the vertical distance from the center of the saddle to the center of the handlebar. However, this measurement does not necessarily have to start at the center of the saddle. The location on the saddle can be changed by altering the saddle reference point.
BikeCAD defines saddle height as the direct distance from the center of the bottom bracket to the reference point on the saddle. The reference point is by default the deepest part of the saddle, where it may dip down in the middle. However, this point can be changed according to the instructions here.