Because there is DC offset (gap of the direct circuit) of the circuit. This value cannot be made as zero. Offset value can be changed to 1/1000 or less of the voltage range (full scale) by turning ON the auto zero function. For example, when the voltage range is 1 Vrms, the full scale voltage is 1.41 V. So, 1/1000 of it is 0.00141 V (worst value)

If the voltage is 0.00141 V, the acceleration is displayed as 1.41 m/s² when the sensitivity value of the accelerometer is calibrated by 1.0 mV/m/s². When you measure the small acceleration, the voltage range should below as much as possible.