Some salt (table salt) has additives like iodine or anti caking agents that you don't want in your ferment because it will cause it to be cloudy (or even more cloudy).
There's also some people who claim the iodine will inhibit fermentation, but studies have shown that it doesn't have much of an effect [1].
Kosher salt contains an anticaking agent. My box of Morton Kosher salt list Salt and Yellow Prussiate of Soda (anticaking agent) and so does my box of Windsor Kosher salt.
Oh you're right! I've got a box of coarse salt as well and it doesn't have any additives - interesting as it and the kosher salt are about the same size.
There's also some people who claim the iodine will inhibit fermentation, but studies have shown that it doesn't have much of an effect [1].
1 - https://www.sciencedirect.com/science/article/abs/pii/S07400...