This would kind of penalize more experienced crafters, who would have a greater and greater chance of not receiving a full stack of 20 arrows/bolts even though their skills increased. And as crazedgoblin said, if it's trivial, it shouldn't matter what you roll (and yes, we still roll on trivial crafting attempts)...you should just get the full stack.
It's a bit counterintuitive to have someone actually improve in a craft and yet still have an equal or increasing chance of losing CNR because of varying yields tied to an ever-increasing range of acceptable die rolls.
Someone with a 95% chance of success would average 10.5 arrows/bolts per successful attempt. Someone with a 25% chance of success would average 18 arrows/bolts per successful attempt. Granted, there will be more successes for the 95% crafter, but I'm sure we've all fallen vicitm to the occasional long string of low die rolls.
This would also require us to program in a special case to the crafting system, which is not necessarily a good thing, nor something we would want to do. This also gets into the question of whether one should get full XP for crafting a partial batch or should that be scaled, causing yet another special case. Lastly, what if we decide to change the number of arrow/bolt shafts in a CNR bundle to 5, 10, 25, 50 or whatever, and thereby change the number of arrows craftable at a single time, then the d20 die roll no longer holds validity.
And what Lalaith said....arrows are consumable, and fairly rapidly so at higher levels. Due to their nature, arrows and bolts (and possibly sling bullets) are about the only things currently craftable in numbers capable of producing a full stack (Batch size of 5, yielding up to 100 arrows per attempt)