Why did Epicor set up Burden Sets in projects instead of passing Burden cost through just like labor cost?

We are in the process of upgrading from Vista 8.03 to the current version of Kinetic. One of the reasons that we chose Kinetic in the first place was the project module. We were expecting to be able to use the T&M and cost plus billing methods, but they don’t seem to include burden the way I would normally expect. (Labor hours times labor rate equals labor cost. Burden hours times burden rate equals burden cost.) I’m pretty sure that kinetic did this with good reason and logic so I would like to learn more about why they did what they did so that I can better utilize the features that are there. Does anybody have a story to share or logic that Epicor developers may have used to set up burden sets the way they did?