"Only the styles set to display in the Quick Styles Gallery on the Home tab are part of a Quick Style Set."
I disagree with that statement.

For me, I guess, the use of Set in your description is problematic. The Style Set is all the Styles. Quick Styles are those displayed in the Gallery. The term Quick Style Set is a confusing term as it melds two specific cases. Quick Styles are not "Set", but can be changed and rearranged.

I would refer to Faithe Wempen and her edition of Word 2013-In Depth, published by QUE (Pearson).

I have a copy of the Word 4.0 Manual and wish I had the ability to run that version to see the leap that's been made to now and how, and maybe why. MS changed things.