Query with different value executes differently

Lets create 2 tables with test data:

1)

create table test_users(
   id serial primary key
);

insert into test_users
select * from generate_series(1,750000);

2)

create table test_posts(
  id bigserial primary key,
  user_id INT
);

CREATE index idx ON test_posts(user_id, id);

INSERT INTO test_posts (user_id) SELECT 2 FROM generate_series (1,30038);
INSERT INTO test_posts (user_id) SELECT 3 FROM generate_series (1,31036);
INSERT INTO test_posts (user_id) SELECT 4 FROM generate_series (1,24032);
INSERT INTO test_posts (user_id) SELECT 5 FROM generate_series (1,93094);
INSERT INTO test_posts (user_id) SELECT 6 FROM generate_series (1,31033);
INSERT INTO test_posts (user_id) SELECT 7 FROM generate_series (1,62063);
INSERT INTO test_posts (user_id) SELECT 724309 FROM generate_series (1,2);
INSERT INTO test_posts (user_id) SELECT 724306 FROM generate_series (1,1);

Now, lets check count of rows and for example post max id per user, from test_posts table:

SELECT user_id, count(*) , max(id) as maxid
FROM test_posts 
GROUP BY user_id
order by count(*)

so, user_id = 724306 have only one post and (in my case) post maxid is: 271299.

user_id = 5 have most posts in table and maxid in my case is: 178200

Now I run this query:

SELECT test_posts.id
FROM test_posts 
JOIN test_users
ON test_posts.user_id = test_users.id 
WHERE test_posts.id = 271299  

explain analyze shows:

"Merge Join  (cost=8.88..9.06 rows=1 width=8) (actual time=146.561..146.563 rows=1 loops=1)"
"  Merge Cond: (test_users.id = test_posts.user_id)"
"  ->  Index Only Scan using test_users_pkey on test_users  (cost=0.42..22808.42 rows=750000 width=4) (actual time=0.012..110.091 rows=724307 loops=1)"
"        Heap Fetches: 724307"
"  ->  Sort  (cost=8.45..8.46 rows=1 width=12) (actual time=0.012..0.013 rows=1 loops=1)"
"        Sort Key: test_posts.user_id"
"        Sort Method: quicksort  Memory: 25kB"
"        ->  Index Scan using test_posts_pkey on test_posts  (cost=0.42..8.44 rows=1 width=12) (actual time=0.007..0.007 rows=1 loops=1)"
"              Index Cond: (id = 271299)"
"Planning time: 0.214 ms"
"Execution time: 146.588 ms"

Now run same query, just use 178200:

SELECT test_posts.id
FROM test_posts 
JOIN test_users
ON test_posts.user_id = test_users.id 
WHERE test_posts.id = 178200  

explain analyze from this:

"Merge Join  (cost=8.88..9.06 rows=1 width=8) (actual time=0.040..0.042 rows=1 loops=1)"
"  Merge Cond: (test_users.id = test_posts.user_id)"
"  ->  Index Only Scan using test_users_pkey on test_users  (cost=0.42..22808.42 rows=750000 width=4) (actual time=0.016..0.018 rows=6 loops=1)"
"        Heap Fetches: 6"
"  ->  Sort  (cost=8.45..8.46 rows=1 width=12) (actual time=0.017..0.018 rows=1 loops=1)"
"        Sort Key: test_posts.user_id"
"        Sort Method: quicksort  Memory: 25kB"
"        ->  Index Scan using test_posts_pkey on test_posts  (cost=0.42..8.44 rows=1 width=12) (actual time=0.011..0.012 rows=1 loops=1)"
"              Index Cond: (id = 178200)"
"Planning time: 0.249 ms"
"Execution time: 0.072 ms"

What causes that difference in execution time ?