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It's because of how quaternions are maid, here is a little tool to see the evolution of the values of Euler angles and quaternion, you could see that (in Euler angles) for x=y=z=0 you can either have the w=1 or -1 (yet it's the same orientation).

For your calculations you should consider the most precise one. Odometry can be wrong if your robot drift, but usually you combine the wheel based odometry and the IMU to have the best value.