For SN8P1900 chip series, when set for single-ended input, it is still differential input for internal PGIA/ADC, because at this time, the positive side of PGIA is user input signal, the negative side is for the ACM, as shown below, Thus, when calculate voltage, user need to calculate the magnification results of PGIA in this way. While choose what method to input, it is determined in accordance with system applications.

Reference document:
SN8P1900 Series Datasheet
Sonix TN009: SN8P1909/8 usage attention