You know, I couldn't find this damned table for some time (after being told to go look at it), because it's on page *5*! Page 5 is for installing instructions, and "what this first menu means" kind of stuff.
Anyway, it explains right there that a 6 is not just rerolled and added. You subtract one, and reroll. So there is no statistical variation in odds between 5, 6, 7 etc. I roll of 5 is 5. A roll of 6 is 5 + (d6). So your odds of success are not affected by the DRN mechanics.
But Endo, it's not a 0-5. It's 1-5, with each roll of 6 simply being another 1-5 on top of that. 0 would just suck.