DC-9 Engine variations and max range

[Flightman]

Confusing...

Example

DC-9-30
2x Pratt & Whitney JT8D-11
48 988 kg (Standard)    13 674 kg   Payload vs. range    1360 NM   Range map    3 530 kg / hr
2x Pratt & Whitney JT8D-15
48 988 kg (Standard)    13 674 kg   Payload vs. range    1360 NM   Range map    3 430 kg / hr
2x Pratt & Whitney JT8D-7
48 988 kg (Standard)    13 674 kg   Payload vs. range    1360 NM   Range map    3 760 kg / hr
2x Pratt & Whitney JT8D-9
48 988 kg (Standard)    13 674 kg   Payload vs. range    1360 NM   Range map    3 690 kg / hr

All have same payload. How DC9 loaded with JT8D-7 can fly far as DC9 loaded with JT8D-15 (eats more than 200kg less fuel - why its not able to fly longer range?)

DC-9-40 are allso different fuel comptions with different engines and all have same range no matter what anginetype used.

Can someone explain?

[Powi]

Missing data. Obviously they should have different ranges.

[Flightman]

Specifications
   DC-9-10    DC-9-20    DC-9-30    DC-9-40    DC-9-50
Passengers
(1 class)    90    115    125    135
Max takeoff
weight    90,700 lb
(41,100 kg)    98,000 lb
(44,500 kg)    110,000 lb
(49,900 kg)    114,000 lb
(51,700 kg)    121,000 lb
(54,900 kg)
Max range    1,265 nmi
(2,340 km)    1,850 nmi
(3,430 km)    1,635 nmi
(3,030 km)    1,685 nmi
(3,120 km)    1,635 nmi
(3,030 km)
Cruising speed    561 mph
(903 km/h)    557 mph
(896 km/h)    570 mph
(917 km/h)    558 mph
(898 km/h)
Length    104 ft 5 in (31.82 m)    119 ft 4 in (36.37 m)    125 ft 7 in (38.28 m)    133 ft 7 in (40.72 m)
Wingspan    89 ft 5 in (27.25 m)    93 ft 5 in (28.47 m)
Tail height    27 ft 5 in (8.38 m)
Powerplants (2x)    P&W JT8D-5 or -7    P&W JT8D-11    P&W JT8D-7, -9, -11 or -15    P&W JT8D-15 or -17
Engine thrust    12,500 to 14,000 lbf (62.3 kN)    15,000 lbf (66.7 kN)    14,000 to 15,500 lbf (68.9 kN)    15,500 to 16,000 lbf (71.2 kN)


they (30 and 40) should fly over. 1600nm