Hate to break it to you mate but your weap dam calculation is not correct.
Damage: 100. Enemy armour: 80. Damage done: 100 – 80 = 20
Damage (with 5% buff): 105. Damage done: 105 – 80 = 25
In my experience with sea demon (i tried using deep sea baby and tidebringer), the armor is substracted before the buffs and the +%crit dam takes effect :/ the weapon glyphs, curse/ascendance add to weap damage before the armor is reduced.
Not to claim i understand whats happening, so armor takes more effect for bigger damages, and smaller for small damages …
Still it might not be the point here, just wanted to share,