MPU6050 positioning problem.

Hi, currently I am acquiring data from mpu6050 sensor. using GitHub - jarzebski/Arduino-MPU6050: MPU6050 Triple Axis Gyroscope & Accelerometer Arduino Library this library. But the problem is when I am changing the position of the sensor and bring back to its reference position the angles are not 0. It is different than 0. What is the problem? How to solve it?