Armour break question -- can't figure out the math

Hi all, appreciate the help in advance.

I have shattering blow -- break 50% of armour on heavy stunning an enemy.

I also have two points in armour break -- each small node gives "break 20% increased armour"

I have tried this multiple times now on an enemy who seems to 200 armour -- when I heavy stun them, 137 of their armour is broken.

I don't understand the math on this. 137/200 is 68.5%.

Obviously, the "break 20% increased armour" isn't additive here, but even if it's multiplicative, it should be 0.5 * 1.2 * 1.2 = 0.72, or 72%. Which would be 144 armour broken out of 200.

What am I missing? Thanks again.