Chess960 puzzle no.20

Symmetry is an interesting subject in Chess960. Sometimes black can play symmetrically, sometimes black can't.

Here is a great little puzzle demonstrating some of the more bizarre situations Chess960 symmetry can throw up:

SP366 Black to play (castling all sides permitted):

Why can't black copy white's play and come out materially equal?

Answer bottom left corner

Answer: because black's queen ends up undefended!

With best play:

3.Bxb8..Bxb1,

4.Bxc7! Bxc2

5.Bxd8! Bxd1

6.Bxe7! Bxe2??

7.Bxf8

3.Bxb8

7.Bxf8