Why does casting AVG(intger_column) as DECIMAL return a minimum of six decimal places?

Question

Consider this query:

WITH Scores (score)
     AS
     (
      SELECT CAST(score AS INTEGER)
        FROM (
              VALUES (0), 
                     (10), 
                     (10)
             ) AS Scores (score)
     )
SELECT AVG(CAST(score AS DECIMAL(19, 8))) AS precision_eight, 
       AVG(CAST(score AS DECIMAL(19, 7))) AS precision_seven, 
       AVG(CAST(score AS DECIMAL(19, 6))) AS precision_six, 
       AVG(CAST(score AS DECIMAL(19, 5))) AS precision_five, 
       AVG(CAST(score AS DECIMAL(19, 4))) AS precision_four
  FROM Scores;

Results:

precision_eight | precision_seven | precision_six | precision_five | precision_four 
     6.66666666 |       6.6666666 |      6.666666 |       6.666666 |       6.666666 

Why do I always get a minimum of six decimal places? Is this documented behaviour?

(I'm running SQL Server 2008)

Answer 1

AVG of decimal always returns a "decimal(38, s) divided by decimal(10, 0)" data type (see here)

You have to cast the AVG result to the desired precision.

Answer 2

I was about to post something similar to Jappie's answer along with this example to illustrate what's happening behind the scenes.

declare @a int
set @a = 0
declare @b int
set @b = 10
declare @c int
set @c = 10

declare @n decimal(10,0)
set @n = 3

select cast((@a+@b+@c) as decimal(38,4))/@n