Good Evening JP5 :) .....that's actually a really good question, and one to which, I don't know the technical answer lol. If I put a good guess to it, I'd say because the base has no excess liquid. Cold-process (my bath soap) and hot process ( what most other artisans are making now) can be made with excess liquid and/or oils, and the ratio of Potassium hydroxide and/or Sodium hydroxide governs the viscosity of the finished product.
Now; as for the microwave. I personally don't recommend melting the soap all the way. Not because it will hurt the soap, but if it gets too hot, you'll burn off the essential oils and lose their good for you skin properties. Most fellas tell me they put the soap into whatever container they're using, and melt it "a little".....just enough for it to stick to the bottom when it hardens back up. That way you're not chasing the puck around.
Another method that fellas tell me about, is to grate the soap with a cheese grater, then press the bits into your container. I think this is a superb idea! It will form to any container you have, and the variation in texture would create a rapid lather. At least to my way of thinking; I should try the two side by side and see if it makes a difference :hmmm: