What is the significance of the blinking white health bars?
Solution 1:
This is the maximum damage of the attack, minus the armour of the enemy. I am pretty sure no other damage sources are taken into consideration (e.g. a tank that is guaranteed to explode).
This mechanic is somewhat bugged. You will notice that in certain scenarios the blinking does not stop, even if an other target is chosen, or if an other attack is done first. What you are seeing is likely the potential damage for a different attack.