Why precision is decreasing when multiply sum to other number

Question

I have encountered with following bug (or feature) in SQL Server.

When I use SUM (*column*) where column has a numeric(18, 8)

Answer 1

a technical explanation can be found at http://social.msdn.microsoft.com/Forums/en/transactsql/thread/233f7380-3f19-4836-b224-9f665b852406