General Grievous Ability Damage Reduced by Offense Down

Not saying this is what happened, but here are some numbers I played around with:
Not knowing how your Grievous is modded, I'll assume he is health modded, but with a Crit Damage primary triangle granting +36% crit damage.

Defense Up provides a 50% armor multiplicative armor increase - so I don't know exactly how that translates into damage reduction. Playing with numbers, a 40% damage reduction for this character fit the model, so that's the assumption I'm using.

Grievous Attack Vs Enemy with No Defense Up: 18,333 dmg
Grievous Attack vs Enemy with Defense Up (-40%): 11,000 dmg
Grievous Crit Attack Vs Enemy with No defense up (+186%): 34,100 dmg
Grievous Crit Attack vs Enemy w/ Def Up (+186% -40%): 26,766 dmg (additive)
Grievous Crit Attack vs Enemy w/ Def Up (+186% x -40%): 20,460 dmg (multiplicative)

For those last two, I'm not sure if the crit damage bonus and damage reduction from armor add together (186-40 = 146% more than base) or if they multiply (1.86*0.6 = 112% more than base), so I showed both.

So in this scenario, you could be comparing a Critical hit versus a character without Defense up to a non-critical hit against a character with defense up. My numbers may not be right, but it's possible.

I'd advise continuing to watch it and see how your crits and non-crits stack up. Plus - see if offense down (or up) have an impact on Grievous' attacks). Good luck!