its because theres a limit to how much can be dropped. when the limit is reached lower rarity stuff is overwritten (older stuff in the case of same rarity). francas drops are 1* so you can guess how that goes.
prime example of this was when people were farming crack from EP6 triggers. they would die before mitra and varuna, and go to campship so they wouldn't get mitra/varuna drops. this meant more luminmech glares, more crack, etc would drop at the end, because mitra and varuna cannot drop crack, and the loot crystal was set to only drop a maximum of 5 solid barriers. their solid barriers would overwrite the solid barriers dropped from fodrus, berouge, and others. same would happen with weapons as well