Supervisor is the person who knows your project and performance best other than yourself. Evaluators usually will show some respect to them and grade accordingly. Moreover, if you knows that there is a big difference between the grading of your supervisor and evaluator, you can appeal, and this will cause some severe problem if the evaulator can not provide enough justification for his/her low grade. Thus usually evaulator(s) won't rish themselves to challenge supervisors decision unless they can find major flaws (e.g. major theoretical flaw, fake data or copying) in your report/presentation.