[biot]

Coming from the user's chest isn't expected, but imagine a reading chair with the lamp over the user's shoulder, behind the chair. Same direction and a common scenario that I can imagine reproducing in software. How often do people place floor lamps directly in front of their chair?

If Apple's HIG says that the light source should always be directly ahead of you, then you are certainly correct. However, if straight ahead is 0 degrees then you can reproduce the opposite behavior (the effects of pitch and roll get swapped) by having the light source at 90 degrees (reading light beside the chair's right arm). The direction of shine rotation on the knob is reversed by having the light source 180 degrees from its current position (reading light beside the chair's left arm).