The evaluation of PhD candidate is done based on his/her thesis, his defence presentation and the oral examination (in some countries). If the examiners do not find so much to argue about, the candidate gets "magna cum laude". In some cases, he may get "summa cum laude".
By attending some PhD examinations and reading the theses, I could not find the key factor, which distinguishes between these two academic honours. The publication list was not a criterion as well. Note that I am talking about examinations in the same faculty of the same university.
Since there is no concrete evaluation of the PhD examination, how examiners decide to give "summa cum laude" instead of "magna cum laude"? In other words, what can the candidate do to make this difference?