Sum Function Yielding Bad Results

I've inherited the following code:

SELECT TOP 100 PERCENT dbo._Options_Demographics.GradeLevel, dbo._Options_Demographics.Gender, dbo._Options_Demographics.Ethnicity,
dbo._Options_Demographics.Age, SUM(dbo._Options_Demographics.Enrollment) AS Enrollment, dbo._Options_Demographics.OptionsYear,
dbo._Options_Demographics.OptionsMonth
FROM dbo._Options_Confirmed INNER JOIN
dbo._Options_Demographics ON dbo._Options_Demographics.DistrictCode = dbo._Options_Confirmed.DistrictCode
GROUP BY dbo._Options_Demographics.GradeLevel, dbo._Options_Demographics.Gender, dbo._Options_Demographics.Ethnicity,
dbo._Options_Demographics.Age, dbo._Options_Demographics.OptionsYear, dbo._Options_Demographics.OptionsMonth
ORDER BY dbo._Options_Demographics.OptionsYear, dbo._Options_Demographics.OptionsMonth, dbo._Options_Demographics.GradeLevel,
dbo._Options_Demographics.Gender, dbo._Options_Demographics.Ethnicity, dbo._Options_Demographics.Age

which returns this data (the enrollment value is 4 times the correct value)
grade gender eth ageenrollyearmonth
8FemaleBlack1618112003December
8FemaleBlack1716212003December
8FemaleBlack187932003December
8FemaleBlack18P2872003December



If I remove the sum function and manually add, my totals are correct ...
SELECT TOP 100 PERCENT _Options_Demographics.GradeLevel, _Options_Demographics.Gender, _Options_Demographics.Ethnicity,
_Options_Demographics.Age, _Options_Demographics.Enrollment, _Options_Demographics.OptionsYear,
_Options_Demographics.OptionsMonth
FROM _Options_Confirmed RIGHT OUTER JOIN
_Options_Demographics ON _Options_Demographics.DistrictCode = _Options_Confirmed.DistrictCode
GROUP BY _Options_Demographics.GradeLevel, _Options_Demographics.Gender, _Options_Demographics.Ethnicity, _Options_Demographics.Age,
_Options_Demographics.Enrollment, _Options_Demographics.OptionsYear, _Options_Demographics.OptionsMonth
ORDER BY _Options_Demographics.OptionsYear, _Options_Demographics.OptionsMonth, _Options_Demographics.GradeLevel,
_Options_Demographics.Gender, _Options_Demographics.Ethnicity, _Options_Demographics.Age

I've been looking at this for too long and I know that is something very trivial ...

William