I've noticed this several times now in EA that mobs can apparently shoot through Floors.
Fighting the Spiders in the Cavern of the Blighted Village: Spiders shoot from the lower are throught the ground of the upper floor to hit my units.
During the fight vs the caved in drow guy in the underdark the goblins can shoot upstairs. My group was hiding up the ladder to right of the cave in and mobs literally shot me from below whereas using the same angle I was not able to attack them due to the "path being blocked".