Any time you do a comparison such as =, <>, or LIKE in a where clause (or inner join), you are implicitly excluding rows where that column is null, since null is never equal to, not equal to, nor like any value. Null doesn't even equal null -- that's why we use IS NULL or IS NOT NULL rather than "= NULL" or "<> NULL".
So if you need to include the nulls, you might want to use something like
AND ([DISPOSITION] <> 'Test' OR [DISPOSITION] IS NULL)