why doesnt a twin tip carve as well as a flat? and why does it release more easily? is it particularly on softer snow that it releases more easily because of extra float?
The short answer is the geometry of the transition zone -- twin tips tend to have mellower transition zones, if other things are equal in terms of ski construction, shape and effective edge. You can make a very mellow squaretail and you could make a true twin tip (not twin shape) that worked as a race ski, or as a practical joke even one that was more nervous than a race ski.
Actually twins can be kind of funky in deep snow -- squash tails actually behave better, though with the newer skis this is less of an issue for other reasons. Basically the shovel on the tail can cause issues with the way the snow flows off the ski.
I'm not qualified to comment on the physics of it, but i think it's just due to less edging power in the rear. In addition a lot of twins tend to be less stiff with less camber, and designed for a more forward mounting, all detracting from carving attributes. I'm sure theres more to the early release