Visual diferente no jtable e cabeçalho do jtable

public class LayoutTela extends JFrame implements ActionListener {
	Container c;
	JPanel cardNorte,cardCenter, cardSul;
	private JLabel lblMatricula;
	private JTextField txtMatricula;
	private JLabel lblNomeProfessor;
	private JTextField txtNomeProfessor;
	final static String TXT1 = "Card 1 em norte";
	final static String TXT2 = "Card 2 em Sul";

	public LayoutTela() {
		super("Layout");
		c = getContentPane();

		
		c.add(registro(), BorderLayout.NORTH);
		c.add(tabela(), BorderLayout.CENTER);
		c.add(botoes(),BorderLayout.SOUTH);

		this.setContentPane(c);
		this.pack();
		this.setVisible(true);
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		this.setSize(400, 400);
	}
	
	private JPanel registro() {
		cardNorte = new JPanel(new GridLayout(3, 2, 5, 3));

		lblMatricula =  label("Matricula");
		cardNorte.add(lblMatricula);

		txtMatricula = textField(40);
		cardNorte.add(txtMatricula);

		lblNomeProfessor = label("Nome do Professor");
		cardNorte.add(lblNomeProfessor);

		txtNomeProfessor = textField(30);
		cardNorte.add(txtNomeProfessor);
		return cardNorte;
	}
	
	private JPanel botoes(){
		cardSul = new JPanel(new BorderLayout());
		
		cardSul.add(botao("Enviar"), BorderLayout.EAST);
		cardSul.add(botao("Limpar"),BorderLayout.WEST);
		
		return cardSul;
	}

	private JPanel tabela() {
		cardCenter = new JPanel();
		cardCenter.setLayout(new BorderLayout());
		
		cardCenter.add(modeloTabela().getTableHeader(),BorderLayout.NORTH);
		cardCenter.add(modeloTabela(), BorderLayout.CENTER);

		return cardCenter;
	}
	
	
	
	public JTable modeloTabela() {
		
		Object[] colunas = { "ID", "Nome", "Conceitos" };
		Object[][] valores = new Object[3][3];
		for (int i = 0; i < valores.length; i++) {
			valores[i][0] = "Teste Coluna 1";
			valores[i][1] = "Teste coluna 2";
			valores[i][2] = new Boolean(false);
		}
		DefaultTableModel modelo = new DefaultTableModel(valores, colunas);

		return new JTable(modelo);
	}

	public JLabel label(String nome) {
		JLabel lbl = new JLabel(nome);
		return lbl;
	}

	public JTextField textField(int tamanho)  {
		JTextField txt = new JTextField(tamanho);
		return txt;
	}
	
	public JButton botao(String nome){
		JButton button = new JButton(nome);
		button.setActionCommand(nome.replace(" ","_" ).toLowerCase());
		button.addActionListener(this);
		
		return button;
	}

	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		
	}
}