Add user capability and check against it

I just realized the following:

// Call the current user info
global $current_user;

// Add a capability
// and set the 2nd argument ($grant) to false
$current_user->add_cap( 'some_cap', false ); 

Now I’ve successfully added a capability to the user, but denied her/him the access to this capability.

If I now check with $current_user->has_cap( 'some_cap' ); I’d get a true in return. The same goes for current_user_can( 'some_cap' ); as this is only a wrapper for the has_cap functions arguments.

Q: Where is the 2nd argument used? Did I get something wrong?