decode produces same result for missing and not missing

this query: 

select decode(field_that_is_missing, missing, 'ok') from default

will produce the same results as this query:

select decode(field_that_is_missing, not missing, 'ok') from default

and this query:

select decode(field_that_is_null, null, 'ok') from default

will produce the same results as 

select decode(field_that_is_null, not null, 'ok') from default

furthermore:

select decode(field_that_is_missing, valued, 'ok') from default

and

select decode(field_that_is_missing, not valued, 'ok') from default

throws a syntax error