I think you mis-read which is what I did...I think he was saying normally 15ppm but then noticed not where it should be...meaning higher.
My tank is 300gal, has 2x30 -60gal auto changes and 1 30%water change per week with zero nitrate water, I also have a deltec nitrate reactor dripping out zero nitrate water....my nitrate often gets to 40ppm....I honestly think people are not testing correctly or they are erring on the lower side or just plain kidding themselves with what their tanks are running at.
If fed decent amounts these fish pour out waste, a deacon filter turns this in to nitrate, nothing to be ashamed of but I think people underestimate the levels and how quickly they can build up...zero to sixty in 1 week is easy