You seem to be right on this one, I will have to investigate further. Quick check on logs shows this one only pops up every 10.
Nynja readies Exenterator.
Records of Eminence: Deal 500+ Damage.
Progress: 40/200.
Nynja readies Exenterator.
Records of Eminence: Deal 500+ Damage.
Progress: 50/200.
But between those two Progress checks there are easily more than 10 Exen's dealing over 500 damage. So my earlier post of checking every WS hit 1 by 1 still seems to be valid.