Measurements of IMU (accelerometer and gyroscope) are drifted or with too much error

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I placed my drone on a table (i.e, at rest) and save (using a ROS topic) the IMU readings:

As you can see, there's a lot of drift or error. I would expect the $x$ and $y$ acceleration to be 0 (I know about the drift, but this is not something that keeps growing, it just has a bias) and $z$ 9.8. What could happen? Before I measure everything, I called the topic flattrim. And I would expect also a better performance for the gyroscope

What can I do to make if work better? If I change the covariance matrices, would it work with less error and drift?

The funny thing is that the velocities (I suppose it integrates the accelerometer readings) are noisy, but their mean is approximately 0:

The covariance matrix is 0 for everything