Yeah, the logic behind it is pretty simple. http://www.wowhead.com/spell=84621
10% crit block after a parry. So the more mastery you have the more block you have and the more of those blocks are already crit blocks.
This scales well with additional mastery and with SB usage, making it more likely in some cases that you'll crit block when blocking than normal block. Basically smoothing out incoming damage even more as your blocks are more likely to be 60% blocks than 30% blocks.
And yeah, as Teng said, it's kinda hard to figure out XD I think RAWR is the best bet? I know Koji has done a lot of modelling and work on HtL value compared to parry's increased DR.