Two majors factors I think most people don't account for when they are practice-testing (other than nerves and getting no sleep the night before):
1. The test is at 9AM. If you are groggy at this time of day normally, that is bad.
2. The actual test has 5 sections, not 4. (Ignoring the essay at the end.) That's 35 extra minutes of doing problems, and the section that is not counted is usually section 2 or 3, when you are presumably less tired, less frazzled, and more able to answer questions correctly than you would be toward the end. On the real test, you have to push yourself through an extra 35 minutes of problems and try to do as well on those last problems as you would during practice, in order to match your practice score(s).