It is just all these programs started in the early 2000s, so the new numerical series. Seriously people would have to be psychologically sick to put product number (like eastern egg in SW) in the supposed manner, either in a way to humiliate themselves or to insult opponents (such as supposed J-35).There is a good chance of that happening if the FC-31 enters service with the PLA.
The reason that J-20, Z-20, Y-20, and H-20 all have 20 within the designation is supposedly that China was twenty years behind the state of the art tech when those projects were conceived. Additional variants within the same generation will carry similar designations.