Board Thread:Questions and Answers/@comment-5139868-20130126141916/@comment-1398253-20130126144250

Because they're not sixth rangers. They're 4th and 5th rangers (that's why Chopper's Gokaiger counterpart is Ahim and not Gai). The only ones who count as a sixth without being that number are AbareKiller (core team had four rangers) and Go-On Silver (she's technically the 7th, but arrived at the same time as the true sixth, Go-On Gold).