To play devil's advocate here, the difference between throwing a grenade up and throwing a grenade at a land-bound opponent is pretty minor. Against a land-bound enemy, rarely are you going to throw the grenade in such a way that all forward momentum is arrested... If the grenade passes through the horde in the time after it 'hits' but before it detonates, your hit would become a miss. Further, if there was time between hitting and detonation, the horde could conceivably attempt to return the grenade to the Space Marine... It seems that the grenade attack rolls already assume you are 'cooking' the grenade so it detonates on contact. A miss indicates either that your timing was bad or that your throw was bad.
Throwing at a flying horde is no different, but if you miss, you'd expect the scatter to be downward instead of a purely random direction. The -10 for a flying opponent seems adequate to cover the difficulty of lobbing a grenade generally upward instead of generally outward.