Look at it as OCV -DCV >= 3D6 -11. That is your character has an ability to attack of 'OCV', the defender has an ability to defend of 'DCV'. You want to perform within your capabilities, just like a skill, but if the opponents are equally capable you want to succeed approximately half the time, so we bias the roll to zero with its average of 10.5 rounded to 11 giving a slight benefit to the attacker.