Hey I played Animal Crossing a lot on the Gamecube as a kid. I know that everyone and every online source refers to this as just Animal Crossing or Animal Crossing for Gamecube/GCN. I do, however, remember noticing the Population: Growing when spending time looking at the box and instruction manual while my brother and sister played (we only had one Gamecube).
I think its meant to be more of a cute introduction to the “Animal Crossing” game title on the box, where the full title reads “Welcome to Animal Crossing | Population: Growing!” On the box, but leaving the game title as “Animal Crossing”.
This might be a Mandela for you, but its also something that I noticed myself as a kid. Hope that helps bring some clarity to your experience.