I don't agree with this. Theoretically, from RP point of view, years pass, you start as a child/teenager and then grow older as you rank up, thus adding to your resistance. Besides this, when you fight you also train your body, exercise is the key of improvement reaching a rank like Jounin or even Kage, it means you have a very vast past-experience. "It ain't about how hard you hit. It's about how hard you can get hit; and keep moving forward." This obviously can't be true if all people would have the same endurance.
EDIT: I just read your last post, the reply to lolernoob. This game tries to empathize with real life, so of course you can improve... what you call "health points". You do this in real life as well. Those "health points" represent your endurance that I spoke about, above. When your HP reaches 50/100 it doesn't mean you are half alive or half dead. You are just like before just that if you get some more hits you will succumb. So yeah, one can get more hits than another if he is trained for this, and yes you do can train for this.
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