Good to know, miss Jane.. makes perfect sense as most people can't go further down than at the approximate height of the barbell when it has a 45lbs/20kg plate on (or proper Olympic disks that start out at the same height.. sometimes even for 'sissy' weights of 5kg/11lbs!).
You can do a real non-SLDL from lower down but you'd need to squat down more.
I'm currently trying to get my legs stronger and squat down on purpose to deadlift.. this has the (weird?) effect of making me weaker on that lift, since my back & hips are stronger than the legs.
Thanks again for the explanation gals!
Now.. we need more ... err cowbell