From 925: "if the player is wielding a single weapon with no shield, or a two-handed weapon, and has a high enough Unarmed Fighting weapon skill, he will receive a DV bonus of half what he'd get from said skill if he were to fight unarmed."

The half DV bonus when wielding a single weapon and nothing in the off hand makes sense - you are parrying with your free hand. Examples of this are not uncommon. However, it makes no sense to me that the half DV bonus should also be granted with two handed weapons. In that case, both your hands are full. It seems like this behavior has been in the game for a long time, but it looks like a bug or a weird inconsistency to me.