Sigil Fire / Staff Abilities
Looks like a pattern… have you tried proccing the Sigil on the Thief with non-auto-attacks?
Tested with a guild thief with close to the same power level and all his attacks were proccing it around 1400.
It’s not just sigil of fire. There’s a vast array of things like lightning rod, static discharge, sigils of fire/air, … That seem to have different damage ranges when procced by different sources. I’ve posted it in the bugs section but I’m pretty sure noone monitors that forum.