sql query sum bringing back different results - sql

Question

I have the following two queries below, the Total is coming back different, but I am adding the sums in each of the query the same way. Why is the total coming back different?
select [Total Children] = (SUM(demo.NumberOfPreschoolers) + SUM(demo.NumberOfToddlers) + SUM(demo.NumberOfInfants)),
County = co.Description
from ClassroomDemographics as demo
inner join Classrooms as c on demo.Classroom_Id = c.Id
inner join Sites as s on c.Site_Id = s.Id
inner join Profiles as p on s.Profile_Id = p.Id
inner join Dictionary.Counties as co on p.County_Id = co.Id
where co.Description = 'MyCounty'
Group By co.Description
select [Number Of DLL Children] = SUM(cd.NumberOfLanguageSpeakers),
[Total Children] = (SUM(demo.NumberOfPreschoolers) + SUM(demo.NumberOfToddlers) + SUM(demo.NumberOfInfants)),
County = co.Description
from ClassroomDLL as cd
inner join Classrooms as c on cd.Classroom_Id = c.Id
inner join Sites as s on c.Site_Id = s.Id
inner join Profiles as p on s.Profile_Id = p.Id
inner join Dictionary.Counties as co on p.County_Id = co.Id
inner join ClassroomDemographics as demo on c.Id = demo.Classroom_Id
where co.Description = 'MyCounty'
Group by co.Description

Answer 1

Just a quick glance over the two querties, I would presume that:
inner join ClassroomDemographics as demo on c.Id = demo.Classroom_Id
in the second query is excluding results that are in the first query, therefor the aggregated values will be different.

Answer 2

Your join to the Classrooms table is joining with an extra table in the 2nd query.
Query 1:
from ClassroomDemographics as demo
inner join Classrooms as c on demo.Classroom_Id = c.Id
Query 2:
from ClassroomDLL as cd
inner join Classrooms as c on cd.Classroom_Id = c.Id
...
inner join ClassroomDemographics as demo on c.Id = demo.Classroom_Id
My bet is that the ClassroomDLL table has less data in it, or has rows with a null for one of the join criteria columns, either of which could exclude rows from the results and throw your aggregate totals off.