When you buy a doorknob, are the keys inside different from the other packages in the store?

Yes BUT, there are only so many tumbler combinations which can be made for a particular lock so there are lots of locks made for the same key tooth combination. Manufacturers try to mix up the shipments so that there are only a few locks with the same pattern sold in the same area. It is possible that two or more lock in a shipment to the same area have the same key/tumbler combination.