N_live_tup value is less than actual row count in a table

I am noticing this wierd  behavior and not sure if this is how Postgres actually works.

I have a test table with 100000 rows and I deleted 20000 rows. So the current rows are 80000 and dead tuples should be 20000. After autovacuum completed, the dead tuples were gone.

When querying the pg_stat_all_tables, it returned n_live_tup value as 63945. Why is it less than actual row count 80000?

I read some articles and it is suggested to analyze manually to bring that n_live_tup number to actual row count. And it did work. After manual analyze, the n_live_tup is showing as 80000.