No, they are not measurement error. The moment of inertia of the earth changes over time in irregular ways (e.g, melting glaciers, etc). Even a major earthquake

No, they are not measurement error. The moment of inertia of the earth changes over time in irregular ways (e.g, melting glaciers, etc). Even a major earthquake can show up in the earth’s rotation. Leap seconds account for the accumulated difference in the rotational period from time to time.