Yaw angle of lse_xsens_mti behaves weirdly

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I ran the lse_xsens_mti driver on my robot . I had it turn 360 degrees at a uniform velocity, and computed the Yaw angle output by the driver data in two ways:

1. Using the tf::getYaw() function on the quat formed by the orientation.x,y,z,w values output by imu/data.

2. By atan2(w,z)*2 on the z and w values.

In both cases, the Yaw angle didn't vary uniformly with time as the robot turned. What could be wrong?