Hey I get what you’re saying and I was thinking the same for some time now but it looks like the “real” life bar is actually the dark red one, the bright red is just the part that is not affected by bleeding.
So when it’s full dark red it just means all his life is affected by the bleeding condition.
And about the feeling of having the mob living more than expected because of the bleeding effect, I think it’s just related to your lvl. Like try on a new level 1 character, any auto-shot will affect the “dark red” bar, the monster isn’t staying alive because of the bleeding. I think at some point when you level up, in a bleeding spec you do so much damage by bleeding and so few raw damage with other spells that it “feels” like you just have to wait.
I don’t know if I made myself clear, english isn’t 1st language !