Why Does One Work, But Not The Other?

I have the following queries. The first returns the 'Unknown' row, the second works the way I would expect. Are my expectations wrong? Can someone describe for me what is going on?

Thanks




Code Snippet
select *
FROM
SynonymComFinancialCategory b
LEFT JOIN TT_FinancialCategory a
ON
a.FinanceGroup = b.FinanceGroup
AND a.FinanceCode = b.FinanceCode
AND a.Finance = b.Finance
AND b.Finance <> 'Unknown'
WHERE
a.FinanceGroup IS NULL
AND a.FinanceCode IS NULL
AND a.Finance IS NULL










Code Snippet
select *
FROM
SynonymComFinancialCategory b
LEFT JOIN TT_FinancialCategory a
ON
a.FinanceGroup = b.FinanceGroup
AND a.FinanceCode = b.FinanceCode
AND a.Finance = b.Finance
WHERE
a.FinanceGroup IS NULL
AND a.FinanceCode IS NULL
AND a.Finance IS NULL
AND b.Finance <> 'Unknown'