How does damage work in Far Cry Primal?

I've been playing Far Cry Primal for a while now and I don't understand how the damage system works. I find it confusing that you have a set of hit points, but you don't lose them in discrete amounts - lots of things do fraction of a hit point worthy of damage to the player.

But what's really confusing to me are the damage stats of weapons. My bow says it deals 3 points of damage, while my spear does 2. Yet I can kill human enemies with either two arrows or one spear (not considering headshots).

On top of that, information sources such as the fandom wiki are no help. The entry for the elder mammoth says it's got five points of life and does five damage. I have evidence in contrary, because when they hit me I lose only two hit points. But worse still, there is this wooly beast which I've named Prickney Spears after I personally turned him into a pin cushion and he refused to die. I counted and there were five spears in it, so should've been at least 10 damage.

What am I not understanding about the way you count numbers in this game? Because this all seems like caveman math to me.