(I'm Selene)
The reason for the low momentum is the way you throw a grenade, in an arch. If you're JUST about to throw it, your hand is in a claw-like state, and moving downward. Depending on the death, you would probably end up throwing it downward or slowly forward. Also, if you're in the last few frames of the throw animation, you DO throw the nade. There's also a few frames after the grenade is 'thrown'.