If you look at the RE;Flex disc mounted on the board you can see that it's setup to allow more of a natural board flex than with a traditional mounting disc.
It made sense to me when I viewed it and considered other options. But, I'm not all that experienced with riding tons of old, or for that matter, new tech.
IT just seemed to make sense to me.....
I think the diff. is so small, unless you're riding every day and paying attention you wouldn't even notice it.