It's directly related to your skill points; if you have a low explosive points they won't be effective. You can test it out with magazines.
I find it funny how the animation cut scene for throwing an explosive may show it yards away but It may or may bot do damage.
I know it has to do with skill points, but does it really make sense to take off a significant amount of damage because I'm not super adept at using explosives? if I can shoot it and it hits someone and it explodes, I don't think I should be penalized for not investing a bunch of points into it. maybe an accuracy penalty, maybe increased risk of jams, i dont know, but i dont think taking off large amounts of damage is the answer.
I have never used a frag grenade or any sort of high explosive, but I guarantee that if I threw a frag or shot an RPG, it would blow something up just the same and cause the same amount of damage.