You might want to emphasize this (percent of body weight) is an age old rule of thumb and can vary greatly depending on the individual horse's metabolism.
Well, it's the average horses needs for maintenance and low level work. Of course, there are be horses with different metabolisms, however the 2% feeding is mainly at horses at good weight. Eg, the norm. I would guess that is easy to understand. If you have a horse at weight loss, or obese, or a skinny rescue, feeding plans need to be made together with the vet to make sure there are no problems. MOST horses will be even slightly over weight if kept at 2% BW feeding with no work.