I don't really get it either. However, the distance covered at 68T while accelerating by 4 knots would be much greater than that covered while accelerating by 4 knots at 44T. Perhaps this is why there is a larger speed reduction at the lower weights? The 12 knots at 44T results in a more significant distance reduction than 4 knots (or less) would because you are accelerating much quicker. At high weights you cover a large distance for every extra knot because your acceleration is so poor. My mind works in terms of distance when it comes to corrections, so ignore the speed value and think of the distance recovered by the reduction?
Don't know if that helps!!
CP