Say you are a Warrior. And say Warriors get on average 5 Hit points per level. Your first level up, you got a whopping 16 points and are now way above average. So, on your next few level ups, since you will be above average, your chance to get more than a point or maybe two is slim (or maybe none). That is how I heard it explained anyways.
The other thing that affects hit points is Fortitude - the higher it is the more hit points you should get (still constrained in the above parameters).
Tools for your Wizardry(r) toolkit