Why autvacuum is not started?

I have running Postgresql 9.4 and... if i have a table with following
configuration:
autovacuum_vacuum_scale_factor=0.0,
autovacuum_analyze_scale_factor=0.0,
autovacuum_vacuum_threshold=1000,
autovacuum_analyze_threshold=1000,
autovacuum_enabled=true

Why autovacuum is not started if the table has more than 1000 inserts???

first i have setted autovacuum_vacuum_threshold to 10,000 inserts (it
is normal each 2 hours ), then to 5,000, 3,000 and finally to 1,000 but
autovacuum is not triggered but autoanalyze.

"select relname,last_vacuum, last_autovacuum, last_analyze,
last_autoanalyze from pg_stat_user_tables where relname like 'sta%';"

last_vacuum=>2017-01-05 10:40:34.228633-06
last_autovacuum => null
last_analyze=> 2017-01-04 15:02:47.438715-06
last_autoanalyze=> 2017-01-09 10:35:51.391114-06

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