After getting Parrying LV2 from the Training Tree, damage reduction on block surges from 20 to 520 when the skill description states that it should only receive a 100% increase.

The skill says "Damage reduction while parrying: 100%."..so 100% damage is reduced when Geralt parries with the sword.

Actually, the value that skill adds, in vanilla, reads <damage_reduction_block mult="false" always_random="false" min="500" max="500"/>. Damage_reduction_block is the only stat used to reduce damage by blocking - they don't differentiate between a block and a parry, in this regard (a riposte is simply a counter-attack, that automatically blocks the incoming attack and any other attack that's attempted while you're performing said counter-attack).

The skill's description says that it will decrease incoming damage from blocking by 100%, so that's probably why they specified the value as high as 500 - nothing in the game deals that much damage in one hit, so it pretty much guarantees that no damage will seep through while you're blocking/parrying. It might look a bit silly if you read the character screen, but as far as I know, the value number is as intended; the skill is supposed to make you impervious to damage when successfully blocking.

Thats for clearing that out.. :)