Those two trinkets are going to be relatively close in power for an Elemental shaman – variations in the fight you are doing or small differences in the model being used to rank them could make them shift a little in relation to each other.
In general, the Ara-Kara Sacbrood can rank slightly lower in multi-target situations because some of its budget is tied up in a single-target direct damage effect.
Thus trinkets that give all stats like the Empowering Crystal sometimes scale up better to multiple targets, as all those stats will keep working for you on all available targets.