After so many stacks the amount of armor you reduce per stack becomes miniscule compared to the first 1-3 hits. If i understand correctly the amount of armor reduced is from current armor level. For example on heroic sion begins with 1080 armor.
After first armor shred his armor is reduced to 810.
Second 607.5
Third 455.625
Fourth 341.71
Fifth 256.28
Sixth 192.22
Etc.
So as you can see the amount of armor "removed" is reduced tons and after 4th/5th time you may not see any further damage boost (or too small to notice)