Why did the A.300 have a lower number than the A.304 despite being more advanced?

The Aero A.300 was a Czechoslovak bomber that first flew in 1938. Despite what its numbering might suggest, it was an improved and refined version of the earlier Aero A.304. Aero designed it to meet a requirement for a bomber and reconnaissance aircraft, intended to replace the locally built Bloch MB.200, already obsolete by 1937. The A.304 itself, a transport/bomber, formed the basis of the new aircraft's design. The prototype made its first flight at Prague-Kbely airport on 20 April 1938.

It was a twin-engine, low-wing monoplane of conventional design, with retractable landing gear. It was powered by two Bristol Mercury IX radial engines delivering 820 hp each, driving de Havilland-Hamilton two-pitch three-blade propellers. It carried a crew of four and an internal bomb load of 1,000 kg (2,200 lb). Defensive armament consisted of three 7.92 mm vz.30 machine guns. Overall, the aircraft is described as a highly refined development of the A.304.

Only one prototype was built. Although the aircraft showed promise, the Munich Agreement and the ensuing Czechoslovak national crisis halted the project's development and production.