Something that just occurred to me is that they might simply be understaffed. Remember this is only 10 years after the Minbari blew up nearly everything that could fly with an Earth insignia. We don't have any hard numbers, but who knows, it might just be that Earth just doesn't have enough Starfuries to make sure each and every ship has a full compliment. Your average ship might carry a half-size flight group, while a ship headed into battle might end up with more on board (hence the 35-36 size).