This is the cohesion level system, something that is level-upped with time and that gives health bonus (as well as other bonuses) to ALL characters that a player has. The higher cohesion level, higher are the bonuses.
That's why a low-level character can have more health than usual, the person you saw must have a high-level main character (or at least, played the game long enough to actively push their cohesion to higher levels).
For example, I personally have a 101 cohesion level (my main character is level 60 HM 22), so my freshly new characters will begin the game with bonuses compared to players who doesn't have cohesion at all.
Hope this helps.