Hello, I need some help understanding how mastery gives block chance.

Currently I am at:
5156 Mastery = 8%(base) + 14.47 (from mastery rating) = 11% DR from SOTR / 22% more block chance / 22% attack power

my block however is 18.47%.

My first thought was that 18% was my base, but as soon as I un-equipped a piece with mastery the 18% dropped.

When naked, my block chance is 10% and mastery 8%.

I have no points in block % from the artifact weapon.

So...
10+22 = 18.47 doesn't add up
10+14.47 = 18.47 doesn't add up
10*1.22 = 18.47 doesn't add up
10*1.1447 = 18.47 doesn't add up


Is there a tooltip bug?