iPhone的加速计校准了吗?重力测量的变化取决于方向

I'm doing some test with iPhone 4S accelerometer. If I take the raw data in Z-axis (telephone rest over desktop) I get an acceleration 9.65-9.70 m/s2 (after g conversion by 9.8261). But if i have the telephone resting over edge, the measurement of the accelerometer value in the X-axis is so different, aprox. 9.80-9.85 m/s2 (after the same g conversion). My question is, if the gravity is the same, why this difference? It is not callibrated?I'm doing some test with iPhone 4S acceleromete