That should never happen. No idea why it happens though, when I hiltslash people with long weapons all day with no problem.
It has to do with the animation and the way the collision point is calculated.
The engine measures the distance from the right fist to the collision point. This is somehow then compared with the weapon reach stat. When you have a long weapon, this ratio of distance and reach changes slowly in terms of absolute distance along the blade. With a very short weapon, every cm makes a huge difference. Since collision detection is not perfect and works in discrete intervals, with short weapons you will get more extreme readings and very often effective "0" distance hits. The damage modification will also be more like full damage or min damage.
The animation between different styles (1H,2H, poles) also has an effect. Animations that extend the hand forward can in principle have the hand inside the target when it hits, while a different animation will hit farther away from the grip even though you face the opponent from the same distance. I do not know if 1H is more prone to this, but I rarely glance with 2H even if I am close enough to lick the nose of the other guy. Not thecase with 1H and 2H-poles.
Short weapons should have had in creased minimum damage, so daggers would almost ignore this distance vs. weapon reach damage modification mechanism. It was not introduced with daggers in mind.
