Nunquam Non Paratus
It's a really a function of a fully loaded magazine being difficult to lock into the magwell due to spring pressure. Loading to 28 keeps an even number of rounds in the magazine making it easy to pull magazine and check that a round stripped as well as avoiding the seating issue. How much harder it is to seat a full magazine varies by rifle as well.Why, they are engineered to hold 30 and have extra room to depress the follower when the cover is attached. There is no reason to download a PMAG.