I've noticed that when making up poultry brining solution, table salt leaves the water slightly but noticeably cloudy even after it's fully dissolved. (Noticeable, that is, in a very large pot half full of two gallons of the solution.) Kosher sea salt dissolves completely clear.
The table salt must contain particles of something that doesn't dissolve in water, perhaps a drying agent or some similar additive. I would find it plausible (though so far unsupported by any actual evidence) that the surfaces of those non-dissolving particles could be a more favorable environment for microbes than the surface of a salt grain would be.
Respectfully,
Myriad