With the blunted tips, the contact points are moved further out. The contact points on the Proto are much closer to the tips than on a non-blunted board, so your effective edge length is longer than comparably sized boards.
To my non-blunted Evo:
Not the best pics to demonstrate the difference, but hopefully you can get the point from them. But, I laid my 158 Evo beside snowklingers 153 Proto and they were virtually identical length contact point to contact point.