Basically, you have a rating which translates into a point, that point then determines your dodge and parry reduction.
For example: Lets say 100 rating = 1 expertise which = 1% (fictional numbers but I don't play melee any longer so go figure). If you have 95 rating, you don't quite manage to get that 1 expertise point, which means you dont get 1%, you also don't get 0.95%. You simply get 0%.
Mastery (at least for Holy) does not do this. It is a percent increase based completely on the rating, though the character sheet shows points similarly to Expertise does, it is EFFECTIVELY POINTLESS. Haha.
For Holy - Mastery Rating = % shielding, period. No point conversion is involved.
I don't care if you already knew this, this is to inform the people that do not know this already. Even if you have 10.92 Mastery you are getting 100% out of your rating, so no worries! :D