ITG3205 sensitivity scale factor symmetrical problem

We use ITG3205 in our new products, as the datasheet, the typical value of the sensitivity scale factor is 14.375(LSB/dps), but we found sensitivity scale factor of the X axis is not symmetrical.
Our testing method is below:
Step 1. Get zero-rate level of the ITG3205, we collect 100 samples of gyro data and then average those samples as the zero-rate level.
Step 2. Spin the pcb board and do the integral of the output data(raw data subtrate zero-reat level) of gyro to get the spin agnle. We spin 90 degree clockwise and then spin 90 degree counterclockwise around one axis, and repeat sever times.
Idealy, as step 2, the intergral angle of output data should be 0, but when we testing X axis the intergral angle is not zero and increment every repeat. For example：
1. Spin clockwise 90 degree around x axis, the intergral angle is 85 degree;
2. Spin counterclockwise 90 degree around x axis, the intergral angle is -5 degree;
3. Spin clockwise 90 degree again, the intergral angle is 80 degree;
4. Spin counterclockwise 90 degree again, the intergral angle is -10 degree;
4. Spin clockwise 90 degree again, the intergral angle is 75 degree;
5. Spin counterclockwise 90 degree again, the intergral angle is -15 degree;
......
We found 6 samples(in 11 samples) X axis have the same problem. Use the same test method, Y and Z axis works well. Is it normal or we made any mistake? Please help! Thanks!