Roid damage is handled a bit differently for Krieg than you may expect (particularly when it comes to his Action Skill) to better support the gameplay goals the team had in mind when designing him.
Krieg's Action Skill uses a custom damage formula that adds Roid damage after the multiplier from the action skill. Skills that give a bonus to melee damage (Empty Rage, Silence the Voices) is applied the Roid damage like other modifiers.
This change was required to keep Krieg balanced while using Roid shields. Prior to the change, Krieg's power level could vary wildly depending on equipment such as a Roid shield. Had this calculation bonus not been changed for Krieg, his overall power level would have been made much lower to compensate and his play styles would not be nearly as satisfying an experience.