The wording of the property is a little awkward, but it is intended to represent 1/+5, the "+" always representing the enhancement level required to penetrate.
Damage reduction is typically expressed in the form "20/+3" or "+3 soak 20 damage". This indicates the amount of damage ignored (20) and the power of the weapon needed to penetrate the effect (+3). Thus a creature with 20/+3 damage reduction ignores the first 20 points of damage from any weapon that does not have an enhancement bonus or attack bonus of +3 or greater. Some feat-based damage reduction is expressed in the form "20/-". In this case, all physical damage is reduced, regardless of bonuses on the weapon.
In fact I changed this in the official tlk a year ago, but I guess that was after the last hak update. You'll see a better wording after the next hak update, I suppose, whenever that is.
