MAX9611's internal ADC is 12-bit ADC. As with any other ADCs, it has gain and offset error. In addition, the current-sense amplifier has some gain and offset error. When precise measurements are required, the device needs to be calibrated at zero and full-scale input voltage.
Regarding the time-varying measurement when the inputs are disconnected, it is because the RS+ and RS- are floating. Please try measuring after grounding both inputs. Alternatively, this issue may be avoided via software by ignoring the results when inputs are disconnected from the end system.