How could DPGS possibly know how many people passed? I have an account with that site and no one has ever contacted me to find out how I did. That's the second program I've seen that has posted some highly questionable pass rates.
You posed a very good question. I went searching for the answer. From their FAQ: "After gathering more than 500 student survey responses from August to October 2016, we calculated that over 99% of our students pass the Aeronautical Knowledge Test on their first attempt." Seems reasonable.
I do however like how you (Gold Seal) have a conditional money back guarantee attached to your course.