Ah, combust would play into it. 20% bigger SS. I'm still slowly working on my DPS set, haven't started working on my VP tank gear.
The logs I'm seeing on WoL show Combust as about 4.2% with SS at 21.3% - so it's more like 1/5th (25%)... of course I was looking at the top logs, maybe they're cheesing it somehow (or just lucky string of SnB procs later in the fight for 1m hp vengence SS combusts overhwelming earlier SS?)

