I took the CISSP a good 6 years ago, back in the days when its still 6 hours 250 questions format. So I am not sure what is the exact format now. I have 2 colleagues who took the exam recently, and here's what I understand and I might not be entirely correct.
You start with 100 question and the CAT system scores you as you answer those questions. If you have answered enough questions correctly, you stop at 100 questions.
If the exam continues on after you've answered 100 questions, it would have seemed that you have not answered enough questions to pass the exam. Hence, additional questions is thrown to you to increasing your passing threshold.
However, there is a possibility that the exam stops at 100 questions and you fail. It meant that you have answered majority of the questions wrongly.
Irregardless of it, you won't know which question is graded or not graded. Best is to study and put all you have on it.
I passed CISSP exam a month ago, and I had to complete 150 questions. Some of my co-workers said they passed the exam with just 100 questions, and some of them said they passed the exam with more than 100 question but less than 150 questions. One of my friends said he passed the exam with just 97 questions two years ago. I believe 25 questions are beta questions and they don't count. Therefore, I believe 700 is the score for 75 questions.