MS Access Rounding Precision With Group By

Question posted 2009 · +5 upvotes

Why doesn’t the average of the score of an employee of each month, when summed, equal the average of the employees score (ever)?

Average

SELECT Avg(r.score) AS rawScore
FROM (ET INNER JOIN Employee AS e ON ET.employeeId = e.id) INNER JOIN (Employee AS a INNER JOIN Review AS r ON a.id = r.employeeId) ON ET.id = r.ETId
WHERE (((e.id)=@employeeId))

Returns 80.737

Average By Month

SELECT Avg(r.score) AS rawScore, Format(submitDate, 'mmm yy') AS MonthText,  month(r.submitDate) as mm, year(submitDate) as yy
FROM (ET INNER JOIN Employee AS e ON ET.employeeId = e.id) INNER JOIN (Employee AS a INNER JOIN Review AS r ON a.id = r.employeeId) ON ET.id = r.ETId
WHERE (((e.id)=@employeeId))
GROUP BY month(r.submitDate), year(submitDate), Format(submitDate, 'mmm yy')
ORDER BY year(submitDate) DESC, month(r.submitDate) DESC

Returns

Average Score : Month 
81.000 : Oct 09 
80.375 : Sep 09 
82.700 : Aug 09 
83.100 : Jul 09 
75.625 : Jun 09

I know 80.737 is correct because I have tallied up the records by hand and done the average. But the average of this table (at 3 decimal places), is 80.56 which is too far off. Does group by mess with the rounding at each step?