As I understand it, butterfat is so highly prized by the dairy industry (because it sells for a higher price), that it backs up in the marketing system and storage space must be found for it. (It was reported in the news that they even use caves.)
Then, when they are ready to use some of the storage butter, they combine it with fresh; salt it to mask any rancid flavor; and send it to market.
If you know differently, please tell.
Also, the salt they use is not likely to be whole Sea Salt
Long ago, I read that sometimes people butter their food to get the taste of salt.
I don't know how true this is.
I do know that butterfat is BIG business. The biggest business that industry has got. (Cheese is cultured butterfat, etc.)
You may also be interested in this post (and thread) in the Parasite support forum...
I eat quite a bit of butter, and have all my life, with 35 years off for margarine. Gloom.
Sometimes we have even bought sweet (unsalted) butter...which is likely more expensive because it hasn't been 'cut' with storage butter.