The reason that there's more heat in this windrow is the fact that there's microbial activity and chemical activity that's taking the organic matter and breaking it down into humus. Composting is the process. The end result of the process is humus. Sitting in Witt's truck is a book that answers that question. Nature works as a system.
Nature is complex.
Still, Witt does his best to simplify it. It's what the soil uses in order to make an environment that gives you healthy plants. Humus is the essence of what healthy soil is. Composting is something nature does. Nature breaks down organic material, turning it into humus.