Wind
Coff, Sorry I took my eye off the ball.
Wind or really the difference between forecast and actual has the greatest influence. hence the need to get closer to the ground.
There is also variability in parachute opening times depending upon so many things. Wight of the load, Air speed, Air density, parachute porosity, floor angle and position in the aircraft (exit time).
On the J using a system called PADS (you will find a good clip on You Tube) we were able to do from 12K what we could at 800 ft using standard 1 tons and a wet finger for the wind.