Good movie but for some details

I've noticed that on the entrance scene, the "human" clan tried to ambush a bison's group. When they realized the humans are there, the bison advanced to ram the humans. So the humans threw their spears and this... made the bison go back and jump down the cliff! WFT??
At least if the spears were on fire this would be more credible but they aren't... that's a fail!
What do you think about this scene??
Anyway, I like the movie! It seems pretty entertained for me despite this kind of details.