symfony2 : kernel.debug is always false

symfony2 : kernel.debug is always false

I use assetic to bundle css and js files together and I've noticed that
even in debug mode they are bundled. That should only happen when
kernel.debug is false.
So I've tried the following:
original :
assetic:
debug: %kernel.debug%
force to false:
assetic:
debug: false
force to true :
assetic:
debug: true
When I force to false, files are bundled. When I force true, files are not
bundled. When I live %kernel.debug%, files are bundled.
Why is %kernel.debug% equals to false even though in app_dev I've set it
to true:
$loader = require_once __DIR__.'/../app/bootstrap.php.cache';
Debug::enable();
require_once __DIR__.'/../app/AppKernel.php';
$kernel = new AppKernel('dev', true);