The interface between the bar and stem is moving up and back along the steerer axis, just as if you were adding spacers under a standard stem. A given length & rise (in degrees) of stem results in an effectively shorter cockpit the higher you move it up.
The "reach" part of the numerical designations for the SC RXL stems is derived not from a horizontal distance between the headtube and the effective bar center, but a direct line between those points. So, no, it's not Reach as understood here at Slowtwitch, and perhaps shouldn't be labeled as such. Why did we?
Because the numbers associated with each stem are an attempt to put their function - that of taking up space between the frame and bar - into a more familiar context. People are used to thinking of stems in terms of length and (degrees of) rise. Rather than list each unique angular measurement, we chose to show direct-line length and vertical rise.
I can see, then, why the stem labels can be confusing. It's not that the #'s are incorrect per se, but they are not necessarily helpful...and if the user tries to use them at face value to derive the chart logic on their own they can end up with paradoxical results. The original 2-D working names for each stem (low-near, medium-far, etc) were really - IMHO - the more accurate representation of what each one was: the macro-adjust structure filling in the vertical and horizontal space between the headtube and the bar. They don't really need numbers any more than the frames do, so long as the logic built into the fit chart is grounded in physical/numerical reality and points you to the most appropriate answer for your fit needs.
Thanks for digging into it the way you did. It'll spark some discussions internally on how we present that chart going forward.