This is a subject I never seem to completely sort out. I am still quite sure a 20% polyester blend will not shrink more than any other fabric.
Various laboratory tests of wool fibers and wool fabrics have several times concluded it's neither water nor temperature that causes shrinking, but these two factors in combination with tumble movements, wringing, stirring, garment rubbing against itself will make the fibers crimple together. It has something to do with the shell like surface on each hair, it catches on another and the result is felted wool of various degrees. As I mentioned this is how they found a way to make super wash and shrink free wool too, they treat the shell like surface of the wool fiber with resin or they rub them off mechanically.
When we buy wool as yarn or fabric it has already been through several washes involving water and soap. Spinning of the yarn or thread is done after the cleaning.
I wonder what the explanation for steam shrinking will be, because it doesn't quite fit the regular theories. That said, I know some use all kinds of detergents on wool and doesn't tell much difference, but I know pure wool needs a mild detergent suitable for wool to stay soft and geneally stay the same quality it had when it was new. Stains, grime and dirt needs to come off, and occation we sort to what ever needed to get things clean. So there's never are complete never either.