One contributor to the "better scaling" with fire is the trinket slots. Specifically, the
[Phylactery of the Nameless Lich] from Sindragosa. Since this trinket provides no benefit to Arcane, the "BIS" arcane gear set uses an i258 level trinket.
Also, for most classes (including fire spec), Sindragosa's trinket is currently modeled as stronger than the Dislodged Foreign object (at equivalent gear levels, i.e., i264 compared to i264 and i277 compared against i277).
The benefit of running an i277 Phylactery and i277 DFO (compared to the same DFO and a Reign of the Dead) accounts for a fairly significant portion of the difference (perhaps as much as half).