It's an Intervention mechanic. The game doesn't want you to be able to have 1 health and a bunch of mana, then take massive damage and end up fully healed. So, any excess health past "death" is subtracted from your Intervention health. But you also shouldn't die from a single hit with mana remaining, so you end up with this.
Basically, you will ALWAYS survive damage as long as you have Intervention, but if you take too much damage you'll end up here.
I learned this the hard way at level 23 on Iron Man. If you have 0 health but still have mana remaining, intervention won't activate, so you die permanently.