Because the P-400 was the version of the aircraft built for export. It had slight differences from the P-39. I'm not sure the casual observer could tell the difference. Provost

The P-400 had a 20mm cannon firing through the propeller hub rather than the standard 37mm cannon. As a result, the barrel sticks out much farther than the 37, which is only slightly past the spinner. Another difference is the number of exhaust stubs on the engine. The '39 has something like 6, while the '400 has something like 12. (These are guestimates, as I'm away from my books, but close enough to tell the difference) SC- .

Other differences were pit layout and oxygen system for the aircraft. IIRC, these were designed for the British, and had systems compatible for the RAF. This certainly caused logistical problems for the US units equipped with P-400s, unless they were stationed with RAF or RAAF units. I remember reading that the 67th Fighter Squadron at Guadalcanal could not get their oxygen bottles filled by the Marines, due to a lack of equipment.

I also believe the P-400 did not have a supercharger which gave it really horrible performance at high altitude.