They do not just let all the chickens and ducks run together and collect the eggs. Generally when any hatchery just sells an assortment direct to customers, it means they don't have enough of each breed hatching to guarantee large numbers of a specific breed. When they sell an assortment to a store, it means that the store wanted to offer an assortment. The hatcheries generally can't band each bird as they box them, when selling assortments. It's not that they don't know which breeds they are, up until the time they box them.Hoover's supplies the local TCS.
They get purebred chicks until September then it is just chicks and ducklings. No breeds. If that is the case it means they just turn all chickens / ducks together and collect the eggs . The produce will not be pure.