Entity Framework / SQL Server strange decimal division behaviour - sql-server

Question

I have a table in my SQL server 2008 R2 database which includes two nullable decimal(16,6) columns. Let's call them column1 and column2.
When I try to run a Linq query against the entity generated from this table:
Table.Select(r => new Foo
{
Bar = (r.Column1 + r.Column2) / 2m
}
);
I get a System.OverflowException if column1 + column2 >= 15846. The message of the exception is only:
Conversion overflows.
With a bit of trial and error I've managed to make the query work with the following:
Table.Select(r => new Foo
{
Bar = (r.Column1 + r.Column2).HasValue ?
(r.Column1 + r.Column2).Value / 2m : 0
}
);
However, I was wondering if anyone could explain what was going wrong with the initial query.
Edit
The first query generates this SQL:
SELECT
1 AS [C1],
([Extent1].[Column1] + [Extent1].[Column2]) / cast(2 as decimal(18)) AS [C2]
FROM [dbo].[Table] AS [Extent1]
With a value of 10000 for both columns, running the query manually in SSMS the result is 10000.0000000000000000000000000 (25 decimal zeros).
The second query has this SQL:
SELECT
1 AS [C1],
CASE WHEN ([Extent1].[Column1] + [Extent1].[Column2] IS NOT NULL)
THEN ([Extent1].[Column1] + [Extent1].[Column2]) / cast(2 as decimal(18))
ELSE cast(0 as decimal(18))
END AS [C2]
FROM [dbo].[Table] AS [Extent1]
Running the query in SSMS returns 10000.00000000000000000000 (20 decimal zeros). Apparently there is a problem when EF tries to convert the first value (with 25 decimal zeros) into a decimal but with the second (with 20 decimal zeros) it works.
In the meantime it turned out that the problem also occurs with non-nullable columns and even a single decimal(16, 6) column. The following ...
Table.Select(r => new Foo
{
Bar = r.Column1 / 2m
}
);
... throws the same conversion exception (with a value of 20000 in the Column1).
Why do those two SQL queries result in two different numbers of digits?
And why can't the first number be converted into a decimal by EF?