I must confess I am no expert on the Ruy Lopez but I seem to remember reading about the delaying of b6.

From McDonald - Chess: The art of logical thinking

5 ... Nxe4 The open variation of the ruy lopez. Black has no intention of holding onto the pawn but instead hopes to gain activity for his minor pieces by getting rid of the pawn that controls the d5 and f5 squares

6 d4 b5 Black decides it is the right moment to block White's idea of Bxc6.

I take this to mean that removing the pawn now is key to getting equality, playing b6 first must therefore have some move order problem 

Interesting