SAT score exams lower based on my grade / other survey questions?

I gave my SAT exam on October 1and got result of about only 1240 of which I am totally unsatisfied. I answered all questions and I was quite sure about getting all maths section correct as I have finished earlier and even rechecked but '6 are omitted answers' written in math section of sat result and I got only 690.I am an international student and I am applying for undergraduate but due to some error, there was '8th grade or below' written in sat ticket. So my question is 'is that the reason for my getting score below than expected?

The SAT does not score the exam differently based on the grade you claim you're in, or other survey questions.

Source: The SAT has a direct mapping of raw score to scaled score. This is published, and there are never comments on this table varying based on survey questions.

It is definitely possible that the mistake of missing questions and answering the survey wrong is on your end. However, if you are sure it's not your mistake (or think it's highly unlikely), you may request a manual SAT regrade (Score Verification Service)