Breda 88 (P.XI)
BF 109 F-1
Potez 633
Bombs mounted under the wings drop in pair to not upset the stability of the aircraft, however bombs mounted under the fuselage are close to the plane’s center of mass by being closer to the aircraft’s roll axis and the engine(s) block. Dropping those bombs singularly will not offset the aircraft balance by much, with smaller bombs having an even smaller effect on the aircraft’s balance.
Having bombs mounted under the fuselage reduce the effectiveness of the plane in terms of payload efficiency and maneuverability, in the case of the Potez 633 dropping two 200 kg bombs at once would achieve the same result as dropping one 200 kg bomb while having the disadvantages of carrying a 400 kg payload.