I would guess the Lambda voltage is just for display purposes. The petrol ECU does the correction and the LPG ECU just follows suit, hence the petrol ECU deciding it's out of range. The trick, which can be done without the Lambda connected, is to switch the car between petrol and LPG at a given speed and load, and see if the petrol injector duration changes. Do that for each of the fuel settings boxes (it will probably highlight the one you're currently on) and it should be closer.
I was thinking the lambda voltage would be good to diagnose the acceleration enrichment, as you can watch to see if it goes rich or lean when you stand on the loud pedal.
Kevin
My Lambdas are not even connected so this supports your theory. The petrol injectors will be connected to the LPG ECU so it can read the timing as you drive, the GM ECU is still in control of the fuel/air mix/timing...