Did you ever get your answer for this?
Yeah, I finally figured it out a while ago, thanks. This is very unsatisfying though, compared to
the beautiful work we've done on BM2...
It seems that in this game it was badly designed. In the previous game there was some logic to this, and in this one it seems that it was just shoved in the last minute. Perhaps there's even a bug that prevents all extras to be unlocked, like the bug in The Dark Eye: Chains of Satinav.
Unless someone can ask one of the designers to dig it up from their design docs, I think it's a lost cause.