Doubt about 'vs Reflex' attacks against unconscious targets

[S'mon]

At 1st glance, I made the mistake of trying to apply common sense (such an underestimated skill) and think that an unconscious character cannot make (at least full) use of his Reflex defense. So I tried to make the hit effective, thinking that a total 18 is not a bad result for trying to hit someone unconscious on the floor.

Rogue's reflex = 24 (his total) - 5 (from helpless) = 19
Boneshard Burst = Rolled 18

He wasn't affected by the attack.

As hinted at above, being unconscious the Rogue grants Combat Advantage, so boneshard is +2 to hit, so 20, so should have hit him.