Hmmm...540-585 represents the pad
stack range, not pad reach, for the lowest stems on the XS frame. If that is a pad reach number it seems really huge, too...570 from the BB would put the center of your pads ~20mm beyond the front wheel axle on an XS, and your shifters out near the leading edge of your tire.
If you followed the measuring convention for the SC that we suggest
here (see Step 1, on page 4 in particular), do you still get 570mm?
Carl Matson