Can someone explain the unit health bar

[dec0y]

I'm a little confused about the unit health bar. It seems to actually be 2 bars, and the right bar is usually smaller than the left one. Could someone explain to me what it means?

 

[Riot]

I think the right bar represents a nonpermanent bonus. Try fortifying your unit on a hill and watch the bar grow - that I think represents the increased strength bonus from 1) fortifying your unit and 2) defensive bonuses.