[blykins]

I'm not the best at explaining things...

Let me try it this way...

You can have an excellent flowing head that performs like doo-doo. You can have a poorer flowing head (read lower numbers) with excellent velocity that performs awesomely.

The reason is because the application varies from case to case and since we're speaking of street engines here that require great throttle response and operate at "sane" rpm levels, you want a head that has higher velocity so that it's easier to make an amount of air/fuel move.