Can someone elaborate on differential cuts?

Ryan… Here's WM's description, which is very complete.

Reverse Differential Hood
Our hoods are cut so that the inside of the hood is made with a larger piece of fabric than the outside. This creates a hood with loose down filled fabric which surrounds your head. That way you can can enjoy warm insulation around your head and face without having to cinch the hood tight. Also, the extra fabric can be adjusted to create a down filled ruff or bill across your forehead.

Differential Cut
Mummy bags are made with an inner shell fabric cut narrower than the outer shell. This is called differential. Its purpose is to give the inner shell a smaller circumference than the outer shell. This eliminates extra fabric inside the bag that could lead to cold spots. All of our mummy bags are differentially cut to promote proper lofting and to protect against down compression when knees or elbows push against the inner fabric. Semi-rectangular bags are not cut this way so that they can be unzipped to lay flat.