understand pg_ndistinct type && Why with(autovacuum_enabled=off) some query estimate 100, some is 200.

source: 
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob_plain;f=src/test/regress/expected/stats_ext.out;h=431b3fa3de1f4f87205e7e27a99ef1cf337f1590;hb=676887a3b0b8e3c0348ac3f82ab0d16e9a24bd43

-- n-distinct tests
CREATE TABLE ndistinct (    filler1 TEXT,    filler2 NUMERIC,    a INT,    b INT,    filler3 DATE,    c INT,    d INT
)
WITH (autovacuum_enabled = off);
-- over-estimates when using only per-column statistics
INSERT INTO ndistinct (a, b, c, filler1)     SELECT i/100, i/100, i/100, cash_words((i/100)::money)       FROM generate_series(1,1000) s(i);
ANALYZE ndistinct;
-- Group Aggregate, due to over-estimate of the number of groups
SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY a, b'); estimated | actual 
-----------+--------       100 |     11
(1 row)

SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY b, c'); estimated | actual 
-----------+--------       100 |     11
(1 row)

SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY a, b, c'); estimated | actual 
-----------+--------       100 |     11
(1 row)

SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY a, b, c, d'); estimated | actual 
-----------+--------       200 |     11
(1 row)

SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY b, c, d'); estimated | actual 
-----------+--------       200 |     11
(1 row)

-- correct command
CREATE STATISTICS s10 ON a, b, c FROM ndistinct;
ANALYZE ndistinct;
SELECT s.stxkind, d.stxdndistinct  FROM pg_statistic_ext s, pg_statistic_ext_data d WHERE s.stxrelid = 'ndistinct'::regclass   AND d.stxoid = s.oid; stxkind |                    stxdndistinct                    
---------+----------------------------------------------------- {d,f,m} | {"3, 4": 11, "3, 6": 11, "4, 6": 11, "3, 4, 6": 11}
(1 row)