Folks, i dunno were you get ur data from, but everything i read about the engine mounted canons of the Bf 109 is the following.
series,mounted gun,ammo load
F-3,MG151/15, 200 rounds
F-4,MG151/20, 150 rounds
G-series, MG151/20, 150 rounds
G-series, MK108, 60 rounds
you must excatly note which MG151 was installed, when the small caliber the 15mm was installed the gun had 200 rounds, if it was the 20mm it had only 150 rounds