I don't know, it's just what the manual says

For my new 55AX, I know I've been enjoying 5pitch props on my previous planes with the 46, so I went to a larger prop diameter with the same pitch for the bigger engine. I imagine this will give me a bit more pull without increasing my speed too much. But of course I'll have to try it and see what happens. I also bought a 12x6 as I heard the plane flies well with that setup.