For imuSensor, the input acceleration is the linear or translational acceleration in the global frame. The output acceleration is the linear/translational acceleration, plus gravity, plus noise all rotated into the sensor's frame. You are seeing the gravity comopnent in your output and it is likely varying because the sensor's orientation is changing.
The orientation input determines how the sensor is oriented relative to the global frame (NED or ENU). So for NED, if the input orientation is quaternion(1,0,0,0) you'd see 9.8 on the Z axis output. But if you change that orientation, you can move that gravity component across the X, Y and Z axes.