Read carefully, I'm a C++ man. Complexity is my business
The point has been discussed variously on That Other Forum. The blocking mechanics is the "default" one used in RQ3 and the original MRQ: I hit, you parry by placing your "thing" between my weapon and your body, and - "whack!" - your "thing" takes the damage instead of you - better have a very sturdy "thing" with high AP. This is exactly what a not-exceedingly skilled shield user does. The roll is basically unopposed. The attacker's skill is less important in this case, since the defender is relying on his shield mass to block, rather than on finesse.
This is not what a skilled fighter does with a weapon. When a competent fighter parries, he attempts to deflect the blow, not simply intercept it. It is a matter of finesse, and the parry is all or nothing. This is better portrayed with an opposed roll, where the higher skill is likely to have the upper hand.