The first TMNT game wasn't titled as such in Japan, instead the NES port of the arcade game (which was renumbered II in the west) got to keep it's title as the first in the series. So because there was already a TMNT 2 in the US, but not in Japan, it got numbered differently in the different regions.

Contributed by Satoshi Kunsai (2093) on Jul 05, 2003. [revised by : lights out 10000 (14006)]. -- edit trivia