WTF am I missing?

[Ikonoclastia]

Trying to get Steel Warrior faction by killing Dorn in North Ro.

Started out Amiable with Indigo Brotherhood.
Started out Threatening with Steel Warriors.

I'm now Threatening with Indigo Brotherhood, so about -1000 faction @ 30 per kill.
I'm still Threatening with Steel Warriors (supposedly +10 per Dorn).

1000 / 30 = 33 Dorns (my pets loving duel wielding his Dragoon Dirks). 33 * +10 to Steel Warriors = 330 which should easily have taken me out of Threatening to Dubious but still no joy.

Edit: Scowling with Indigo now, still Threatening with Steel Warriors.