Again this can involve a loss of accuracy. First FontForge checks to see if the spline happens to be a quadratic already (this would happen if you’d just read in a truetype font, or if a miracle occurred when you generated the spline). Otherwise FontForge will divide the original spline into smaller chunks and try to find a set of quadratic splines that differ from the cubic by no more than one unit anywhere. (Once you have picked two end-points and know the slope at those end-points there is only one quadratic spline possible between the two).