You being one of the top students at your school with that type of grammar really makes me wonder what type of high school you go to... but that's not the issue.
Considering you only use that type of grammar on the internet, I would say that there are a lot of reasons for why your SAT score was below what you wanted. Since you said you "score", I'm assuming that you regularly take SAT practice tests, but if you're talking about your score on the first SAT you ever took, it could largely be due to practice. If you hadn't practiced before, then you should look into it.
If you have, the only thing I can say is make you're not the best of standardized test-takers.