There are various AoE spells in the game that hit or fail to hit targets unexpectedly. This happens with both offensive spells and buffs, I'm sure I see any pattern to it why this happens, I'll list three examples but I think most spells should be reviewed for this problem.

 

The Druid spell Burst of Summer Flame does not hit either wurm in the AoE:

 

 

The Priest spell Armour of Faith misses the BB rogue in the green circle but hits the druid with the Sabre:

 

 

The Priest spell Holy Meditation hits two targets clearly outside:

 

It looks like the calculations are off when doing the AOE selection on varied terrain. Good catch!