Based on my observations, constant requests of ATC for offline AI traffic to either climb or descend is based on two different altitude models. It appears ATC is using pressure altitude corrected for 29.92 above 18,000ft and the AI altitude model is using the actual QNH that is not adjusted above 18,000ft. Real time AI traffic are unaffected since they are not controlled by ATC. In addition, real time traffic will appear on the VFR map when zoomed, but offilne traffic will not.  When flying in low pressure areas, AI traffic will be too high and vice versa.  When I adjust weather to be clear skies ( Standard day ) , then the com[plaining stops as I assume the altitude models are in agreement

Basically ATC is using measured altitude above 18000ft, assuming a standard day model, just as a pilot would observe on his altimeter when selecting 29.92 in the window. The AI altitude guidance model, however, is using true altitude. If this premise is correct then there should be no ATC requests for AI altitude adjustments below 18,000ft.