I think there could be a number of things responsible for what you’re seeing. For example, mounting the gyro IC onto the sensor PCB can affect the factory calibration IC, so you might need to do some additional calibration in your code. Also, are you being careful not to rotate the board more quickly than the gyro can measure?
One of our customers posted a fairly detailed write-up, including code, of using our gyro to track the rotation of his robot. You might find this post helpful:
