Way too many people are on the you can't know anything until you know your official MCAT score bandwagon. But honestly you can judge the range you will score on the MCAT with practice tests. I am sure many people have been surprised by the fact that their MCAT scores did not generally line up with their practice tests, but they are most likely the exception not the rule. Furthermore, an applicant knows their past and how they perform on standardized tests. If you have had trouble with the SAT, ACT, GRE you will probably have trouble with the MCAT as well. If you have performed very well on standardized tests, you have a better idea of how you are going to score on the MCAT.
My confidence arises from the fact that I know I can get an MCAT score that will be good enough. I have a GPA that is good enough to convince admissions committees that I can survive medical school. I will most likely not have amazing stats that are going to wow admissions committees, but they will convince admissions committees that I can make it through med school. The reason why I am so confident about an acceptance comes from the quality of the rest of my application.
The application process is not really that mysterious or unpredictable. The AAMC publishes great statistics that show you where you stand as an applicant
https://www.aamc.org/download/321508/data/2013factstable24.pdf. Obviously if you score lower on the MCAT or have a lower GPA you don't have as good a chance. But if you have a GPA>3.6 and a MCAT>30 you are sitting pretty good. With theses stats you have a 67.7% chance of acceptance. Now if you are well prepared, write quality essays, get great LORs, apply early, and apply broadly (to schools you actually match well) this percentage is going to go up. If you practice for interviews, work through common questions and have good answers, and have a decent personality your chances will go up.
I have all these things in my favor and thus my confidence.
Now if you have a GPA<3.6, and a MCAT<30, the process does become more unpredictable and you could end up needing some luck.