Why Bb3 and why Na5?

Recently I've been looking at a game and this position arose after 7 moves.  What I don't understand is, why was Bb3 played (moving the bishop twice) and then why did black play Na5?  Also, after all is said and done, why didn't black play Nxb3?

Why did he play Ne8?

Nxe4 was threatened followed by d5.  Bb3 does not allow for the fork on d5.  Why Black played Na5 remains a mystery

Isn't this a master game?

That looks like Fischer-Reshevsky.  11... Kxe6 leads to 12. Qd5+ Kf5 13. g4 Kxg4 14. Rg1+, with mate following.  The most complicated line ends in 14... Kh3 Rg3+ 15. Kh4 Qe4+ 16. Kh5 Qg4#.  

The game continued 12... Nc6 13. Qd2 Bxe5 14. O-O, with Fischer eventually winning on move 42.

ah I see, thanks for that!  I've hoped so much for someone to play Na5...but noone ever plays that move hehe.