Reduced Takeoff Area
Jammedstab,
I don't know if you work in the same jurisdiction of the operator in the example you cited, however, Aus CAO 20.7.1B - 12.1.1 (b) allows the lateral expansion of the takeoff area to be discontinued when 0.125xD intersects the RNP containment area (2xRNP).
In plain language, when doing an RNP departure the area for obstacle assessment is narrower, potentially allowing greater uplift as limiting obstacles would not be considered (as mutt alludes to above).
I would hazard a guess that the operator concerned has an RTOW chart labelled "RNP" or it's EFB performance program allows selection of a variable or runway label that accounts for the reduced takeoff area with an RNP departure.
Alternatively, the "RNP" option may only be provided at airports where there is a commercial benefit for doing so. Cairns RWY 15 (when RNP departures were used) comes to mind as an example.