Help Nexus 5 acceleration sensor bias

[Android Question]

I noticed that the acceleration sensor on my Nexus 5 is biased on Z axis: when I put it on a horizontal surface, it show 9.3m/s2 (instead of approx. 9.8), and when I put it upside down it shows -10.3 (instead of -9.8). On a vertical surface, like a wall, it predictably shows -0.5 instead of 0.

Is there a way to calibrate it ? I do a standard calibration (rotation in 3 axes) after every power up, but the accelerometer is still off.