React re render array whereas item key has not changed

Very basic code sample of a list:

class List extends React.Component {
    render() {
        const listComponent = this.props.numbers.map((number) =>
            <Item key={ number.toString() } value={ number } />,
        );

        return (
            <div>
                <button onClick={ () => this.setState({ test: 1 })}>Re-render list</button>
                { listComponent }
            </div>
        );
    }
}

An here is the item:

class Item extends React.Component {
    render() {
        return (
            <div>{ this.props.value + ', rendered time:' + new Date().getTime() }</div>
        );
    }
}

When I click the button, the state is updated so the List component is re-rendered.

However, if my understanding is correct, the items should not be re-rendered since the key item has not changed. But it does re-render since the timestamp is updated.

Can someone explain me why?