With anything alive-like plants-they also produce byproducts that will feed the beneficial bacteria for the nitrogen cycle-as well as have the beneficial bacteria on them.
With live active growing plants-depending on species, number and growing state-Often it can take a long time if ever to see the nitrate reading we normally look for to tell us cycling stage/completeness. Due to the plants using ammonia and other byproducts before conversion-The cycling is still happening-its silent. Plus, you have to remember that the testing products we use only test at a ppm level and we will always have some ammonia in the tank-but at low levels that do not harm the livestock.
Also, remembering that additives used in the tank-like dechlorinator and especially plant ferts can cause skewed test results or false reading.