cover is directional. If the grenade land behind the sandbag and in the middle of your squad it's going to hurt.
Funny story, the hitbox on the things are bit weird so the grenade landing in the middle of the squad where the green cover dots are will cause it to do half damage and cause no losses on its own.
Now if the grenades lands just a bit behind those dots, it'll count as not being in the direction of the cover and probably get a model kill or two more to the soldiers closer to it (but the other guys will probably take less damage from the grenade being farther from them).