Query thinks that IS NULL = " "?

[Banaticus]

SELECT *
FROM [aTable]
WHERE [aTable].[aField] <> 'thing'

The query is returning most of the results where [aField] is not equal to 'thing'. But, it's also not returning results where [aField] IS NULL. Doesn't this mean that the query is thinking that "thing" = IS NULL?

 

[Banaticus]

Wrapping the field in Nz() enables it to return the value correctly -- replacing the Null in the field with whatever it is that Nz() replaces it with. I don't really understand why Null has special properties -- how are we helped by that?

SELECT *
FROM [aTable]
WHERE Nz([aTable].[aField]) <> 'thing'