The scores calculated by practice tests are simply estimates. Your actual score is very likely between these two numbers. The reason has to do with the computer-adaptive aspect of the test, and the fact that some questions, when answered right, do more for your score than others, and some questions, when answered wrong, do more harm to your score than others.
(If you answer a really really difficult question right, it helps a lot, and if you answer a really really easy question wrong, it hurts you more than missing a difficult question.)
The scores you receive are based on the level of difficulty of different problems, and the practice tests do their best to estimate your score, but can only give you an estimate. The actual test has an enormous database of statistical information on each question and the percentage of students who have gotten it right and wrong and their respective scores on the exam. Tricky stuff.
As for the non-GMAT-branded practice tests --
They're great for preparing for the exam: for working on timing, for becoming accustomed to questions given in rough difficulty sequence, and for improving your score. However, they are only an estimate when it comes to the actual score.
The practice test that tends to have the highest level of reliability on the actual score is the one written by the authors of the GMAT. Unfortunately, there are only two of these.
I hope this answered your question, though it's probably not the answer you may have wanted,