[GENERAL] is (not) distinct from

I get strange results:

with a as (select citing_article, cited_article, pubyear, year_cited,  cited_author, cited_title, cited_work, doi
from wos_2017_1.citation
where citing_article='abcdefg'
order by 3,4,5,6,8),
b as (
select citing_article, cited_article, pubyear, year_cited, cited_author, cited_title, cited_work, doi
from wos_2017_1.citationbackup
where citing_article='abcdefg'
order by 3,4,5,6,8)
select distinct b.* from b , a
where
( B.citing_article,
             B.cited_article,
             B.pubyear,
             B.year_cited,
             B.cited_author,
             B.cited_title,
             B.cited_work,
B.doi)
 is distinct from
        (A.citing_article,
             A.cited_article,
             A.pubyear,
             A.year_cited,
             A.cited_author,
             A.cited_title,
             A.cited_work, A.doi)

The result of this query is 28 rows - thus that of b. 
I expected this to be 10.

If I change the query to "is not distinct from" it results in 18 rows

which is what I would have expected.

Regards

Johann.

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