CakePHP 3 - Bad render html when I want to generate a multiple checkbox field

I've read documentation about cakephp 3 on this subject. I would like to create a multiple checkbox field but I've had a few problems in my view.

My view :

<?php
    $option = [
        'Value1' => '1',
        'Value2' => '2',
        'Value3' => '3'
    ];
?>
<?= $this->Form->create(null); ?>
<fieldset>
<legend><?= __('Catégories'); ?></legend>

<?= $this->Form->select('categ_list', $option, [
    'multiple' => 'checkbox',
]); ?>

</fieldset>
<?= $this->Form->end(); ?>

The render :

Preview :

I don't know why cakephp generate this html code. I turned off BootstrapUI Plugin but the problem persists.

I use CakePHP v3 and I'm a begginer.

Thanks !