Most of the small and larger local mills use a concentrate and local grains (if available) when mixing there feeds.
It's more simpler and cheaper for them to do so. I don't know what brand your using but there is a good chance that they use a mixer also.
I use King from L. A. Hearne Company in King City. They do use a vitamin pre-mix and they do add nutrients that are in short supply in the feedstuffs, but they mix and manufacture their own products. I am currently using their soy and corn free non-GMO feed.
A number of smaller companies, including Scratch and Peck also source their own feed, possibly with the admixture of a small amount of pre-mix solution.
But you're right, and if it concerns you, see if they use a concentrate. I believe almost all, if not all, feed manufacturers do add vitamin, mineral, and amino acid balancers to their feeds.