I agree that it is a cheap media, but gives good price/quality ratio when average PI is 15. Your results are made on DVD-R, which is definitely bad media. My tests were on DVD+R.
P.S. I found that good media (average PI=15) are with DVD+R written in inversion colour in a box (inner part of a disc, side where recorder writes), bad media are with "dvd+r" letters without box.