So, ref salt is a bunch of different minerals. In mixing, shipping, storage, ect... different minerals weight different amounts and are different sizes.Please explain stratified salt.
Take a bag of dog food, crumble up half of it and then pour each back in the bag. It'll be pretty mixed up at first, but tank that bag and shake it a few times and drop it on the floor a few more. All the fine stuff will filter to the bottom and the larger pieces will settle out on top.
Salt manufactures use a type of clay dirt to try and bind the minerals and keep them from getting stratified. Mix some salt and leave it in the barrel, you'll have a brown film in the barrel. It's not slime or algae, it's clay.