foo:
  pattern: /foo/{bar}/{car}
  defaults:
    bar: barValue
    car: carValue

Generate a route in a controller:

$this->get('router')->generate('foo', array('bar' => 'barValue', 'car' => 'somethingElse');

In your scenario where a default value is hidden from the url 'barValue' would NOT be added to the url because it matches the default value so the generated url would be: /foo/somethingElse

Now if someone clicked that link and requested /foo/somethingElse and it happened to match the foo route then it would be the equivalent of /foo/somethingElse/carValue

The bar and car param has been mixed up.