Great question and observation.
Today, my daughters would be just as well off with the bag zipped and flat, with the zipper on one side. They are pretty straight sleepers (don't like to be diagonal).
For my son, he is more of a diagonal sleeper and likes to pull a little of the width out. That means only a single layer of sleeping bag in the middle, so he puts a fleece blanket under his butt to compensate.
I wish I weren't at work and could go take a picture of what I'm trying to describe. probably hard to understand in words, but easy if you saw it.
Worded differently, doing it the way I described lets you adjust the width by pulling the top layer away from the centerline. My son makes his wider by doing this, so he doesn't hang over the edge of his UQ.
Clear as mud?
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