How does the horses actually work?

I don't have the recordings but an enemy was standing still while in horse, shot him twice in the head of the soldier with a sniper rifle, he survived, a lot of that damage was absorved by the horse even if the shot only hit the soldier. While if i go on horse, i always die fast but not my horse, like the one taking the damage it's me. How does this make any sense? how can i handle enemies at horse if 2 shots in the soldier head is not enough?