The A/D input that measures the sensor has a circuit that starts at +5V and goes through internal resistance, then out the connector via the sensor and then back via the sensor ground. No other currents than what is needed for a clean input is running through here. The power grounds connect the PCB to the chassis ground in a separate circuit, with it's own associated voltage drop - or in other words, there is no current from the power ground wires running through the sensor ground which means no voltage drop is created because of it.
That's the short version that comes to mind, maybe someone else can chime in if you don't get what I said.
