What you see is what you get. If you think that you could do better under different circumstances, you need to be retested when you are in those circumstances (to get an alternate score). As others have pointed out, that score is affected by autism, depression, lack of sleep and so on.What is the best way to figure out your IQ if you're an Aspie?
IQ tests all measure a variable in intelligence called "g". Nobody knows for sure how to define "g", but it is consistent enough across different IQ test that psychometrists can take a black-box approach to its analysis.
Counselors & educators use this number to facilitate teaching & counseling you.
You referred to your score as an FSIQ. That terminology is usually associated with the WAIS-IV test, which is an official IQ test. It is considered to be one of the most thorough available. Were you tested by a psychologist?