the reason for it is nutrient load. the more fish you have the more variants there are in the water chemistry. if your adding phosphate for example via ferts whilst having big poo machine fish your not going to have any idea whats actually in your water. thats the reason for a low fish load.
personally i'm the opposite i am technically way over stocked but the plants keep the nitrate below 10 consistantly. without the plants my fish would not be too healthy.