Why domain's disk space usage in Plesk is different from size of document root in file system?

Question

Why is the disk space usage in domains list (Plesk > Domains) different from the size of the document root on the disk?

Answer

The web and database files are calculated only for the main domain in the subscription.

The mail and others are calculated for the individual domains that are found inside/under the subscriptions.

An example how the calculation occurs for domains under a subcription can be seen below:

MYSQL_LIN: MariaDB [psa]> SELECT SUM(httpdocs) + SUM(httpsdocs) + SUM(subdomains) + SUM(web_users) + SUM(webapps) AS web, 0 + SUM(mailboxes) + SUM(maillists) AS mail, 0 + SUM(dbases) AS databases, SUM(anonftp) + SUM(logs) + SUM(domaindumps) + SUM(configs) + SUM(chroot) AS other FROM disk_usage AS disk_usage INNER JOIN domains AS domains ON domains.id = disk_usage.dom_id WHERE (dom_id = '111') or webspace_id = 111;
+------+------+-----------+--------+
| web | mail | databases | other |
+------+------+-----------+--------+
| 0 | 4096 | 0 | 430080 |
+------+------+-----------+--------+

Basically, this means that even though there are web files and database files found in the document root of the domain, these files will not count towards the disk usage of this domain. Instead, these files will count towards the disk usage of the subscription's main domain.

Additional information

How disk usage is calculated by Plesk