Duplicate data in the response of a query

Here is the code:     

$conn = new PDO('mysql:host=localhost;dbname=locadora','root','');

$resul =  $conn->query("SELECT * FROM usuario");

echo "<pre>";
$resul = $resul->fetchAll();

?>

And the output looks like this:

array(2) {
  [0]=>
  array(8) {
    ["id"]=>
    string(1) "1"
    [0]=>
    string(1) "1"
    ["id_cli"]=>
    string(1) "1"
    [1]=>
    string(1) "1"
    ["user"]=>
    string(12) "david.santos"
    [2]=>
    string(12) "david.santos"
    ["senha"]=>
    string(60) "I82A1G"
    [3]=>
    string(60) "I82A1G"
  }
  [1]=>
  array(8) {
    ["id"]=>
    string(1) "2"
    [0]=>
    string(1) "2"
    ["id_cli"]=>
    string(1) "2"
    [1]=>
    string(1) "2"
    ["user"]=>
    string(12) "joao.batista"
    [2]=>
    string(12) "joao.batista"
    ["senha"]=>
    string(6) "senha"
    [3]=>
    string(6) "senha"
  }
}

Doubt is because the data is duplicate? I know that if I use "fetch (PDO :: FETCH_ASSOC)" the problem will be solved but I would like to understand why this is happening