Because it was with 87 gals of fuel, as it can be seen via the takeoff weight.
P-39N is too late for the time frame, since it was in production last 2 of 24 months of interest here. Same applies fot the Hellcat.
Again, I disagree on the weight.
Saving 33gal (120-87) would only account for 198#. The N grossed 7650 with 120gal fuel, an additional 198# would have increased weight of the test plane to only 7472# (7274+198), still below published gross weight.
Joe Baugher's site shows the first 100 N models to be within serial numbers 4944 to 5043 and this plane was #4400 (probably a K or L with the new -85 9.6 geared engine). The reduced fuel models started with the 167th example indicating the test model had the full 120 gallons. No K or L models had reduced fuel capacity.
ALL the AAF planes were tested (official performance test) at a weight below their published weights. If you discount this test then you should discount every other test. This indicated they were using average or mean fuel during the flight. Taking off at full gross weight for the test and "correcting" the test weight to reflect average or mean fuel for that flight.
The British "corrected" (their term) the gross weight in their tests to 95% of takeoff weight to reflect the average weight of that flight with average fuel.
Also, in the performance tests the amount of fuel was almost never mentioned, even when they state the number and caliber of guns and the corresponding amount of ammunition.
When reduced fuel is mentioned, specifically one test each with the P-51A and P-38G, both those planes were lighter than published weights AFTER the reduced fuel is deducted.
Again, if you discount this test than you should discount every other AAF test, they were all light.