The Extra Damage Slingbow Augment 3 adds 400 more damage.
Attacking a wildstock, my damage is 1673 using a Titanium slingbow. By attaching the augment 3, I now get 2565 damage. This is 892 more damage. Not 400.
How is the game calculating damage?
If you have a skill that increases a damage modifier like the damage epic or slingbow epic, this percentage is likely applied after the damage of the augment is added.
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Woooooo percentages!!! 
They do everything! From hunger to damage calculations percentages are the answer!
Also with things like this (im not to sure just spit balling) like in league of legends some things are done multiplicitive and some are additive. Could be the reason as well for odd numbers.