Bleeding Rend (IV) doesn't do enough damage

I was fighting some Stag Beetles in the Tree Cave today and was looking at my bleed damage, and Bleeding Rend (IV) doesn't seem to be doing as much damage as it should. The beetles appear to have around 25% bleed resistance (My Flanking Attacks which should do 16-17 damage to them where doing 13-14), but my Bleeding Rend was doing as little as 3 points of damage on some hits when the minimum damage should be around 7, maybe 6 (The minimum combined damage of my weapons is 30).

For reference, my weapons are a rank 76 Titanium Rapier (Min damage 16) and a rank 80 Darksteel Mace (Min damage 14). I have Ambidexterity 4.