I'm curious, why not a 13x6 with a .70 four-stroke?

I have a Venus almost ready to go (after how many months! ) with a Saito .72. The manual (well, the box actually) says to go with a 13x6 if using a four-stroke.

I know that the 12x8 will give more speed, but I always have "heard" that a 13x6 is the "right prop" for a .70-size four-stroke...