It is normal with EVERY auto.
When you put it in park, there is a little latch (it's actually technically a pawl) that extends into a gear, which has a certain number of teeth. That pawl engaging one of the teeth in the transmission is what sets the park - the pawl is mated to the case, the gear on the output shaft.
If, when you shift into park, the pawl is not exactly lined up with one of the teeth, the car will roll forward (or backward) until the pawl engages a tooth on the gear. This is why sometimes it will roll forward an inch, sometimes 5-6 inches.
It's certainly nothing to be worried about - that transmission simply happens to have fewer teeth on its parking pawl gear than others, which means the car will roll more than others until it finds a valley between teeth.
Keep in mind also that when parked on a hill, this pawl is the only thing keeping the car from moving; the entire weight of the car is on that pawl. The greater the incline, the more weight on the pawl. This is why sometimes it can be difficult to shift out of park on a hill. This is also why you are supposed to set the parking brake when parking on a hill, to keep the weight off that pawl (which will snap, and that's an expensive fix!).