Think about it. The base game is still the same, but all sorts of new features have been added and it's been given a new subtitle. This wouldn't be the first time Animal Crossing did this...
That's a really interesting comparison. Welcome Amiibo is kind of like to New Leaf like e+ is to Population Growing but I would say that New Leaf's update may have added even more than Population Growing's update did. I like it when they do that because it keeps the games relevant for longer. I'm also extremely glad that Welcome Amiibo wasn't Japan only! xD