The range quoted by manufacturers is normally the maximum range. This is difficult to achieve in practice as it assumes maximum fuel, which is not possible with a reasonable passenger / freight payload.
As VASI correctly suggests, each aircraft has slightly different fuel burn, and a percentage is applied to the standard fuel calculation to take this into account. Also some aircraft have a lower Basic Empty Mass (BEM) which will allow more fuel for a given passenger load (since all aircraft have the same MTOM) which in turn means more range.
Departure airport runway length / runway in use / weather conditions also has an impact, as a "Performance Limited Takeoff Mass" may be applied which is less than MTOM, meaning less fuel can be loaded for a given PAX load.
So in reality there is a different "potential range" for every single flight on every single aircraft.
I used to love the smell of Jet-A in the morning...