Voltage sensor Calibration
Multimeters are designed to provide a stable response. It would be averaging the readings. 

As far as the difference in voltage reading, if you are using the Arduino board as is the default is to assume a reference voltage for the A/D as being 5.0 volts.(for your calculations) That may not be the case. Also there may be a tolerance issue on the measurement board, but I think the reference for the A/D on the Arduino is the bigger issue.