They don't have to bleed to die. Sometimes it's exhaustion. Sometimes it's handled badly and some of the protective film is removed. That leads to infection and the fish may die days after you caught it.I don't know about the %10-15. If you made the fish bleed badly with your hook or hook it deep and cut the line the infection from the hook or it bleeding slowly will kill it, that is why I love using lures , the chances of just getting it's lip only is great.
Using singers with dual hooks, I've had to end up taking some fish.