Equipment is not the only factor in determining damage reduction due to resistances.
What factor is causing the huge damage reduction in the video, then? The video showed the target character's build. Moreover, damage was identical with or without the stance/oath, suggesting that this is not due to some hidden reduction.
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When you tested with level 1 characters you should have been doing within the listed weapon damage range, with bonuses for a power stance or Oath of Desecration.
This is not the case.
Here's another video, which demonstrates that I am getting damage well below the minimum damage in the tooltip against a zero armor target using both power stance and oath of desecration at level 1:
Edit: Here's a third video demonstrating that the lowered damage is *not* the result of some reduction factor that I have not considered. As shown in this video, the damage with melee power stance / oath of desecration is exactly the same as the damage without. The buffs are doing nothing.