This would be true if you only have one press producing the coins and one rolling/bagging machine. Usually, when the mint is ready to produce large quantities of coins, they install the working dies in the presses. Now, if one working die has some damage or has been reworked and creating a variety, was used for a press say out of 10, then 10% of the coins would be of a different variety. Then, all the coins are sent to mamy bagging machines, making it possible that some bags have more variety coins then others. I hadn't really considered the number of presses in operation at one time, but I guess it depends whether the separate press outputs were combined before being fed into the bagging machine, or whether they were fed in serially.