The part apparently sums VC11 and GND, turning on both bottom switches in figure 7 for the sample. You can see this on an oscilloscope if you look carefully seeing the effect of the voltage divider of the external resistor and internal switch resistances. GND wins for the measurement, so you are effectively measuring VC10-GND.
Figure 7 is the safety measurement system, the cell monitor works similarly with a parallel circuit.