User blog comment:Ivan Yap/Skip Episode III and just title it as Sonic the Hedgehog 5!/@comment-86.133.154.164-20120822055038

If I had to guess I would say the idea came from Sonic 3 and Knuckles which are 2 seperate games that continue the same story and the story conitinues because of Episode Metal which connects the events of Ep I and Ep II so thats why (Only a guess).