Based on your stated assumptions, all of the attackers should have the same mod twpa. Orcs and undead have equal twpa bonuses (minus some sci, I guess) -- but the sci eff penalty of orc isn't nearly enough to explain the gap between your orc and undeads - it looks like about .8 epa and .8 dspa, since you aren't counting elite defense in the def measure.

Give more detail on your calculation. What TWPA numbers are you assuming? It looks like you're assuming less than you should be.

(p.s., I do love 'What the #1 kd is doing is clearly wrong because they have 10+ of the weakest race, and 14-15 of the weakest 3')