NOS (Not Otherwise Specified) is a catch-all phrase used widely in the DSM (Diagnostic and Statistical Manual of Mental Disorders). It is usually placed at the end of some group of diagnosises for the remaining cases that don't exactly fit in one of the previous categories.
Under Gender identity disorder in the DSM is
- 302.85 Gender identity disorder in adolescents or adults
- 302.6 Gender identity disorder in children
- 302.6 Gender identity disorder NOS
Note that the same diagnostic code is shared by children and NOS.
I guess the only answer to this is to ask your therapist how your case does not fit in a standard diagnosis.