This behavior is part of the standard and should not be affecting the results of your query. I recommend you read these 2 articles:

http://www.sqlservercentral.com/articles/Advanced+Querying/gotchasqlaggregatefunctionsandnull/1947/[/url] - which is old, but AFAIK NULL handling by aggregates hasn't changed.

http://msdn.microsoft.com/en-us/library/ms187810.aspx - where the top line states that "Null values are ignored."