Mob farm spawn rate

I have built and tested the mob farm design by Mumbo Jumbo in several worlds. My most recent test is about 130 blocks from the highest ground. I stand on top of the farm which would be about 150 blocks from ground and maybe 1 mob will spawn in 10 minutes. I stand to the side on one end and will get about 1 spawn per minute. But it's nothing like the 10,000+ drops per hour boasted by the designer. Am I just missing something? Because a mob farm is pretty important for a good supply of bone meal and what not.