Poly varnishes are not simply polymerized--they contain some amount of polyurethane resin, in single part varnishes almost always modifying alkyd resin. There just isn't any reason for poly varnishes to contain more thinner (not solvent). If there is a reason I'd bet it is mostly economic, thinner is cheaper than resin, and single part polyurethane varnishes typically aim for a lower price segment of the market. If a varnish doesn't state the resin you can pretty much assume it is alkyd. The other basic choice is phenolic resin, and the varnishes that contain that tell us--ie. Waterlox and Behlen Rockhard. You can also pretty much assume that the oil used as an ingredient is linseed oil, unless you are told otherwise, like Waterlox touts it's tung oil, and Pratt & Lambert it's Soya oil. Sure on the specialty fringes there are other oils and resins, but that's of little relevance to those of us who by finishes from retail sources.