Why does grenade shrapnel go up and out, but not across?

I know that if a grenade is thrown next to you then the best thing to do is to hit the deck, but I see no reason why the shrapnel cant hit you if you

I know that if a grenade is thrown next to you then the best thing to do is to hit the deck, but I see no reason why the shrapnel can't hit you if you hit the deck?
Why does the shrapnel only go like this?

or:I know that if a grenade is thrown next to you then the best thing to do is to hit the deck, but I see no reason why the shrapnel can't hit you if you hit the deck?Why does the shrapnel only go like this?

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