In my opinion the current water test kits we have are so cheep that they have a lot of compromises that make accuracy questionable. I your case is zero the correct reading or is 30 to 40 ppm correct? Somehow you need verify your test result. Many labs out there purchase calibrated test samples and use them to verify their lab equipment is generating correct results. You might want to purchase such a sample. I have looked a the one linked below.It contains packets of water that have 10ppm of nitrate in them and others that have 100ppm. If you dilute the 100ppm packet with an equal amount of distilled water you should get a solution with a 50ppm reading. That might tell you if shaking the bottle is right or wrong.
So the nitrates must be from the end result of the cycle. It makes sense that my tank is high in nitrates, though. I don't do many water changes as I test my parameters weekly and they always came back clean! But now that I've shaken the bottles, it's clear that I've been getting false readings this whole time!
http/www.amazon.com/gp/product/B0085XFTPA?psc=1&redirect=true&ref_=ox_sc_sfl_title_3&smid=ATVPDKIKX0DER