“but there is no way of knowing if the completed cases have the correct weight bullets; right?”
A disciplined reloader would know if the weight was not correct, the extra weight could be caused by powder and or bullet.
Only on the Internet.
When reloading I do not work with WWHUA, If it does not work, I did it. And, yes, it is possible to, under some conditions, mix bullets, one manufacturer has bins of bullets, something like a courtesy to drop-in customers, they have blemished? bullets. The only way they can sell direct to a customer is charge retail or sell blemished bullets at a discount. I am sure individuals are more careful and not as careless as Home Depot shoppers but anytime bins are used things ‘can’ get scattered.