Grade Discrepancy

SELECT P.person_id
, person_name = P.nick_name + ' ' + P.last_name
, Member_Status = L.lookup_value
, P.birth_date
, Age = dbo.fn_Age(P.birth_date,getdate())
, Grade_As_Entered = dbo.core_funct_grade_name(P.graduation_date,getdate(),1)
, Grade_Based_On_DOB = dbo.core_funct_grade_name(DATEADD(YEAR,(CASE WHEN DATEPART(MONTH,P.birth_date) >= 9 THEN 19 ELSE 18 END),P.birth_date),getdate(),1)
, Grad_Year_Entered = YEAR(P.graduation_date)
, Grad_Year_Per_DOB = YEAR(P.birth_date) + (CASE WHEN DATEPART(MONTH,P.birth_date) >= 9 THEN 19 ELSE 18 END)
, P.medical_information
FROM core_person P
JOIN core_lookup L on P.member_status = L.lookup_id
WHERE P.birth_date <> '1/1/1900'
AND P.graduation_date <> '1/1/1900'
AND YEAR(P.birth_date) + 18 NOT BETWEEN YEAR(P.graduation_date) - 2 AND YEAR(P.graduation_date) + 1
ORDER BY P.graduation_date