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I've added a response to the Stackoverflow question. Shortly put, `value-changed` event is being fired when the parent component (`<iron-data-table>` in this case) binds a value to the `value` property of the `<vaadin-combo-box>` after the default value has been set to it. So there is an actual value change going on.
In your use case, it's better to use two-way binding instead of listening to `value-changed` events, as I've explained in my answer in Stack.