Passing the road test requires dedication, but above all it requires the student to be "Cool"! Over the years as a driving instructor, I have seen thousands of students take their tests with the DMV. Although I can tell when someone is ready to test, that does not mean the student will be guaranteed to get the "passing grade" he/she is after.
"Cool" means having confidence and keeping yourself calm. There have been many excellent drivers over the years who had everything going to do well, but small mistakes held them back and brought them back for a second exam. When you stress yourself with the prospect of failure, you shift the odds against you. A easy test becomes an emotional challenge.
I have also seen situations where I was not entirely confident in my student's ability to pass and recommended additional lessons. In some cases, students went straight for the test without following these recommendations. At times, I was right and we would continue working together towards that second test. From time to time, a student who may not have seemed ready, passed nonetheless. A common trait I always identified with the group is having this "Cool".
Many things influence having the "Cool". I believe confidence in yourself is the key factor along with experience and a tiny bit of luck.