Short answer: the weather.
The altitude sensor in the GDX-ACC device is an air pressure sensor with a wide range (-1800 m to 10,000 m) and about a 1% tolerance in accuracy (for absolute measurements, approx. 120 m from any given actual pressure reading). Keep in mind that the relative measurements are quite sensitive (1 m changes are easily seen).
Simple changes in weather also bring changes in air pressure in about a 6% range, which is greater than the accuracy of the sensor.
The value reported straight out of the box may thus deviate from your actual altitude within that tolerance. Like all pressure based altimeters, you must set a starting altitude for the subsequent reported values to make sense. Expect unit-to-unit differences depending on the air pressure when the units were calibrated.
If you are only interested in relative measurements of altitude from starting base station then simply use the zero function in Graphical Analysis, Graphical Analysis Pro, or LabQuest App to zero out the measurement before proceeding with your experiment.
If you want accurate altitudes reported during the experiment, you should use the calibration functionality in the data collection apps to set your actual altitude prior to starting your experiment. For best accuracy, you should reset the base altitude if more than an hour has elapsed since the last time you set the altitude basis; weather induced atmospheric pressure changes, which affect the reported altitude, can occur rapidly.
A reasonable source for your actual altitude is Google Earth or any topographical map of your location. Be sure that you are actually on ground level when calibrating using such an altitude source (a typical single office floor can be as much as 5 meters above ground level).