The problem isn’t that there’s another route that matches the URI. The problem is that you never tell your router to do its thing. 
You need the following code to process the router’s routes and call an appropriate one:
// Get the appropriate route from the Router
$route = $router->run(kirby()->path());
// Return if we didn't define a matching route to allow Kirby's router to process the request
if(is_null($route)) return;
// Call the route
$response = call($route->action(), $route->arguments());
// $response is the return value of the route's action, but we won't need that
// Exit execution to stop Kirby from displaying the error page
exit;