MAP sensor voltage vrs MAP sensor - 2 bar or 3 bar

I'm trying to understand how the values are derived or where they are sourced from for these channels in the scanner. I'm running a 2 bar SD OS and I'm using a 3 bar MAP. I don't have a 3 bar MAP channel available in the scanner (I assume that is because I have a 2 bar map OS loaded). I have the correct MAP linear and Offset values for the 3 bar in the tune and it runs reasonably well. Looking at the MAP voltage value in the scanner, when I convert the voltage to kPa using the sensor linear values the pressure reading comes out about where I expect it to be for the 3 bar sensor, yet with the 2 bar map channel loaded it reads the same as what the 3 bar calculation is. I'm confused with that. How do the 2 bar and 3 bar map channels work? Are they calculated values within the scanner using the map sensor voltage value? Right now I'm unsure of the accuracy of the MAP channel kPa values as it relates to my tune so any clarity on this would be of great assistance. Thanks.