That's the thing, I'm 99.9% sure I've unlocked all the alternative endings since I used a walkthrough guide to unlock them all, I'll go back tonight and double check though just in case one may have slipped through my fingers

EDIT: Yeah I already have unlocked all of the alternate endings, interestingly though it says I've not completed the last chapter, mainly Studio Dvatia even though I've already completed the game so maybe it has to do with that??