NPRs at designated airports can only be changed by the Government, and that would require public consultation, so that can't be the reason for what you're seeing.
An aircraft flying the conventional SID, which strictly speaking is a procedure rather than a routing, can wander up to 1.5km either side of the nominal NPR centreline before it's considered "off-track".
An RNAV1 SID, on the other hand, is a route that approximates the NPR by defining a series of waypoints which, as Heathrow residents have found over the last couple of years, are flown much more precisely, the effect being to concentrate traffic (and therefore noise) over a much narrower swathe than the 3km wide NPR
If you are experiencing flights that are considerably adrift from the conventional NPR, that should be reflected in Gatwick's published track-keeping stats.