DLC no longer owns any power generation plants- they buy the power. The difference is DLC's rates are regulated. The third party rates are not.

When you choose one of these third parties, they essentially go to anyone of a number of generators on the grid and buy the electricity on your behalf. So they may in fact be getting the power from a different source than DLC gets it, and maybe at a cheaper rate than DLC pays. Of course, they'll mark it up a little or take a loss for a few months, and once the initial term expires gouge you.

There used to be some good green suppliers out their; the original Green mountain was one. They've since changed ownership though.