The mastery only raises the Chance to Block Spell Damage up to its maximum, but not beyond.
I have 70% Chance to Block Attack Damage, therefore i would expect the mastery to give me 14% Chance to Block Spell Damage. I'm starting with 58% Chance to Block Spell Damage and a maximum of 68%, so I would expect to see "68% (72%)", but there is only 68% displayed. If I increase my max block with the mastery I then get to 70%.
I added screenshots below: