Board Thread:Naruto Discussions/@comment-30168130-20161120204250/@comment-734582-20161121112004

BeyondRed wrote: So all things considered, the unique properties of Naruto's seal seem like the most likely explanation for why he has a special form and B doesn't, though it sucks we never got any official confirmation. I think Naruto's seal allows him to actually mix his and Kurama's chakra to create the chakra shroud (retaining the Biju inside but leaking chakra out) whereas B's seal (and other seals) do not possess such advanced features (either Biju "essence" in or out, creating biju parts (wings/tentacles) or red biju chakra).

Presumably it works because it's a dual seal, one retaining the Biju essence while the other retains the Biju's chakra.