In previous Marlin development versions the Z Probe Offset was negative for a typical servo based probe. I've pointed out that this did not make sense as when the probe triggered, the extruder tip was ABOVE the bed (ie the Z position is positive). In other words, the Z probe offset (which is really the extruder tip offset) is positive. In the case of the Robo R1 with the Z nut switches, the Z probe offset will be negative. This is because when the Z nuts trigger the extruder tip is logically (but not physically of course) below the bed. Anyway this whole issue has been an ongoing source of confusion. So it wouldn't surprise me to see this change many times until some kind of final version comes out.