is (not) distinct from

When I query table a I get 18 rows.
The same query on table b results in 28 rows.

Both tables have the same structure.

When I export the results to csv-files and do a diff it confirms that all
18 rows from a are also in b. Table b has 10 new rows.

When I combine these queries and use "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.
--
Because experiencing your loyal love is better than life itself,
my lips will praise you. (Psalm 63:3)