Mysql – Strange issue with thesql index cardiality

I am facing a weird issue when trying to understand (explain) a query.

I have this table:

CREATE TABLE `trk_track` (
 `track_id` int(11) NOT NULL AUTO_INCREMENT,
 `track_date` date NOT NULL,
 `track_hour` int(11) NOT NULL,
 `track_createdAt` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
 `user_id` int(11) NOT NULL,
 `action_id` int(11) NOT NULL,
 `url_utm_id` int(11) DEFAULT NULL,
 PRIMARY KEY (`track_id`),
 KEY `track_date` (`track_date`,`track_hour`),
 KEY `user_id` (`user_id`),
 KEY `action_id` (`action_id`),
 KEY `url_utm_id` (`url_utm_id`)
) ENGINE=InnoDB AUTO_INCREMENT=284538 DEFAULT CHARSET=utf8

There are about 200k lines in this table.

But something is very confusing:

1-) First the cardiality of url_utm_id index should be 200, but it shows '3' and after analyze table it shows '493', which is still wrong;

2-) When I try to run this query:

explain select count(track_id) from trk_track where url_utm_id in(185);

it shows:

id  select_type     table   partitions  type    possible_keys   key     key_len     ref     rows    filtered    Extra   
1   SIMPLE  trk_track   NULL    ref     url_utm_id  url_utm_id  5   const   53  100.00  Using index

It is ok because there are only 53 url_utm_id(185) in trk_track.

But if I try to do:

explain select count(track_id) from trk_track where url_utm_id in(select url_utm_id from trk_url_utm where utm_id=102);

it shows:

id  select_type     table   partitions  type    possible_keys   key     key_len     ref     rows    filtered    Extra   
1   SIMPLE  trk_url_utm     NULL    ref     PRIMARY,utm_id  utm_id  4   const   1   100.00  Using index
1   SIMPLE  trk_track   NULL    ref     url_utm_id  url_utm_id  5   db_trk.trk_url_utm.url_utm_id   577     100.00  Using index

This subquery:

select url_utm_id from trk_url_utm where utm_id=102

will return same '185' ID

But anyway it sounds like that it tries to make a join, look in explain and it scan 577 rows, it doesnt' make sense for me.

It should scan 53 also, shouldn't it?