Shadow Grasp is, to my memory, "You suffer X damage from your Shadow Grasp."
Playing with Fire increases the damage done by your spells by 3% (+24/tick) and increases damage taken from spells by 3% (+24/tick).
As a side note, with 3/3 Arcane Instability (+3% spell damage) and no Playing with Fire, I always take 824 damage/tick from Shadow Grasp, no deviation.
Hah, ok, that will do it.
Still odd though. For one, I'm not sure how some people got ticks for other values. What caused Zoea to take 11-15% extra? Darkness? Shadow Weaving? Also, Alayne tells me that AP definitely didn't affect it.
Been trying to figure out the same thing for Scalding Water at Lurker. 3/3 PWF explains that perfectly.
I'm really unsure about the others though. What is weird is that it's not always the same damage -- Zoea illustrates that perfectly. Darkness increases shadow damage done by 10%. Shadowform increases shadow damage done by 15%. Shadow Weaving shouldn't matter, since it's not on her. But you see her max tick is 920 which is 15% more than 800, but she averaged 888 per tick. Very bizarre.
Does Alayne have Arcane Instabilty though? That explains the 824 ticks, but why wasn't it always 824 ticks?
It's the lesser ticks which are really confusing. Zoea's 920 max tick can be easily explained - it's Shadowform. Darkness doesn't apply.
Digging into spell data, it looks like it's a difference between the effect "Apply Aura: Add % Modifier (Damage)" which seems to internally reference a list of things it's supposed to apply to, and "Apply Aura: Mod Damage % ()", which just applies to everything of the school given in the parameter. Darkness and AP call the former; Shadowform, Instability and Playing with Fire call the latter. Oddly enough, this suggests that while AP wouldn't do anything, Power Infusion would.
I'm not very familiar with the WWS tool. Could it be as simple as some people partially absorbing damage?