for a differential cut, the layer of material that will form the top of the quilt is cut wider. the extra width is needed for this top piece of material to curve over the insulation without binding it.
the insulation curves over you. the inner surface of the quilt does, too. say it takes 32" for the inner surface of the quilt to curve over you. add the 1" thick (or however much) thick insulation and the outer layer of the quilt has more to wrap around (you plus the insulation) so it needs to be a little wider or the insulation might be compressed or bound up a bit.
it's like putting trousers and belt on, then magically and instantaneously adding adding long underwear, then finding out you need to let the belt out a notch.
the formula above helps you figure out how much to add. the figures given as recommendations are close enough for a 'typical' quilt width, if you don't want to fool with the math.