It seems that the sensor requires a re-calibration when it is used with the Arduino Expansion Board. Here is a useful link that describes a method to calibrate analog sensors, you might find it interesting: Arduino - Calibration.
Additionally, I would test the voltage analog readings by comparing them with voltage readings using a multimeter. This test will allow you to know if the ADC is properly reading the voltage values.
Checking the voltage readings is what I ended up doing and there was a significant difference. I re-calibrated the sensors and the readings were better.