A fake health bar is a staple of modern dumbed down games. Its purpuse is to trick people into thinking they were close to death, when in reality they had much health to go on. The fist couple of damage will have an higher inpact on the healthbar compared to the damage taken when the health appear to be low. While the heath-bar displays health at 50%, the reality might be that youre character has 80% health left.
It is meant to give the illusion that the game is more dificult than what it is, to make people slow down to heal before they die, or to trick people into thinking it was a fair/dificult fight.
We thing people playing our game are idiots so lets use a fake health bar.