Minecraft diamond axe sharpness?

So apparently you can put sharpness 2 on a diamond axe using an anvil, but you can't put sharpness 3,4,5. You can't combine 2 axes with sharpness 2 and 1 either. Does anyone know why? Or is there any way to do that?

Guest

I'm pretty sure you can do it, don't know what the problem is

dave