Not wanting to be condescending, but is this your first new car? I don't consider myself old whatsoever, but I've already owned 5 new vehicles (out of 8) and they always had lower than advertised mileage for the first ~5000-8000KMs. General rule of thumb is to start analysing after the first oil change
I drive my N55 somewhat spirited and more often than I need to in Sport+ mode, and I still average ~11L/100 combined (21MPG) which still boggles my mind for such a heavy & powerful vehicle. (my old Rainier used to gulp ~20L/100KM average and over 25L/100KM in winter).
Give it time; these awesome engines + 8SPD combos are insanely efficient.