Sensor Offset
The accuracy of a sensor and the offset of a sensor can manifest in a similar way. A reasonably accurate acceleration sensor would measure the gravity of earth when the axis of the sensor was pointed straight up towards the sky.
Determining the accuracy is a little more complicated than this because the gravity of earth actually changes slightly depending where on earth you are and because of sensor offset. If the positive axis of a an acceleration sensor over-estimates the gravity of earth, and the negative axis of the same sensor under-estimates the gravity of earth (or vise versa), there is likely some offset occurring where the center of the axis is slight skewed towards the positive or negative axis.
Note that the gravity of earth is equal to 9.78 m/s^2 - 9.82 m/s^2 depending on where you are on Earth.
We can see that the positive z-axis of the Nexus 4 MPU-6050 slightly over-estimates the acceleration of Earth's gravity.
We can see that the positive z-axis is slightly under-estimating gravity while the negative z-axis is slightly over-estimating gravity indicating there is a small amount of sensor offset occurring (the center of the axis is shifted slightly towards the positive axis) in addition to a small degree of inaccuracy in the measurements. Although the errors are small, in an scenario where you would want to integrate the measurements, say to calculate the velocity, the errors begin to compound and become large over a period of time. There are methods to help correct for the accuracy and the offset, but they are beyond the scope of this article.