It makes perfect sense. Gen Y was the last generation fully born within the millennium that just ended (from 1,000 A.D. to 2,000 A.D.), hence the descriptor pertaining to that millennium. Gen Z was the first generation to be born into the first century of the new/current millennium, and they'll be the ones who are society's elders once this current century closes out. However, members of Gen Z can also be described as "Zoomers" or "iGens" to link the specific attributes representing their generation.