In your 2oz difference example, you're comparing a Nunatak arc to a Nunatak sleeping bag. A quick look shows they're pretty much the same, except that the bag gives a hood and a way to zip it closed. Nunatak is the rare company that offers the same bag in a quilt. For a more apples to apples, you need to take a top of the line quilt, such as the Nunatak Arc Alpinist and compare it to a top of the line sleeping bag.
Compared to the Western Mountaineering Ultralite 20*, both are 20* insulation, both have 2.5" of loft per layer, based on their fill weights, they appear to be the same grade of down fill power. However, since the WM has to fill underneath you, it takes more weight, and the comparable WM bag comes in 9oz heavier.