Maybe the difference is parliament. A dictatorship will have advisors and may delegate limited control to other entities however they are the 'final word' and the only opinion that cannot be circumvented. Traditional monarchies would have also been dictatorships. Modern monarchies delegate significant power to parliament in manners that can't be circumvented by their own will. So I'd say that the difference between a modern monarchy and a dictatorship is a democratically elected parliament that has power beyond the will of the monarch.