That isn't to say that the filament is not wet, but there are not really bad "batches". When the filament is made, it is about 0.2% moisture is pulled onto a reel where only a small portion of it is exposed to the air at the surface. It is NOT usually spooled and weighed right away. When it is spooled, in theory, the outside layers of that reel will have a higher moisture level than the inside of that reel.
I have recieved filament in a vacuum sealed bag that had condensation on the bubble of the inside of the center of the spool. Even with this vacuum seal and the silica packet, water can still pass through the plastic into the material on the spool (albeit very slowly). This is why filament can get "old" sitting on a shelf. In a very humid environment, the filament can still get wet in storage.
It is assumed that it is end user responsibility to dry it when it gets wet, even if it comes that way from the manufacturer.There really is no way to prevent it before delivery to the consumer, not without expiration dates and a substancial increase in the material.
You do want to remember 3 things.
1)Avoid dehydrating filament unless it is actually wet.
2) Get as close to the glass transition temperature of the material as you can while drying it. With PLA that is typically 50-55C
3) DO NOT over dry filament. Maximum 6-8 hours and only if the filament is actually wet as seen from visable results obtained from making a flow cube.
A small amount of moisture is actually necessary. The water along with other additives act as a plasticizer to keep the material flexible. Generally speaking, the water will evaporate first, but the other additives will go as well the longer you dry it. Hense why you only dry if it needs it.