What does the revision_default column do in the paragraphs_item_revision table?

I am trying to debug a situation where

* I have a node with nid 1
* I have one revision to that node, vid 2
* I do not have any prior revisions in any language
* That revision (vid 2) has a field which has a paragraph
* The paragraph in that field has id 2759 and revision_id 539075
* I have deleted previous revisions of nid 1
* I have repeated used drush err:purge paragraph until there is no orphan paragraph left
* My database is not shrinking in size as I expect.

I have found that the database is keeping other old revisions of paragraphs, and that there are fields which are linked to those paragraphs. For example:

select * from paragraph_revision__saison where entity_id = 2759;

shows me entries where the revision_id is not not 539075, which to me mean they are fields pertaining to orphan revisions of paragraphs.

I can surmise that the latest revision of paragraph 2759 is 539075 by typing:

MariaDB> select * from paragraphs_item where id = 2759;
+------+-------------+---------+--------------------------------------+----------+
| id   | revision_id | type    | uuid                                 | langcode |
+------+-------------+---------+--------------------------------------+----------+
| 2759 |      539075 | my_para | 364f6dc5-272a-4874-9935-2e5d57c5bcd7 | fr       |
+------+-------------+---------+--------------------------------------+----------+
1 row in set (0.000 sec)

I can see that the database also contains other revisions of paragraph 2759:

MariaDB> select * from paragraphs_item_revision where id = 2759;
+------+-------------+----------+------------------+
| id   | revision_id | langcode | revision_default |
+------+-------------+----------+------------------+
| 2759 |      492302 | fr       |                1 |
| 2759 |      495197 | fr       |                1 |
| 2759 |      538319 | fr       |                1 |
| 2759 |      538823 | fr       |                1 |
| 2759 |      539075 | fr       |                1 |
+------+-------------+----------+------------------+
5 rows in set (0.000 sec)

My question is, in the paragraphs_item_revision table, I would expect to see something like this:

MariaDB> select * from paragraphs_item_revision where id = 2759;
+------+-------------+----------+------------------+
| id   | revision_id | langcode | revision_default |
+------+-------------+----------+------------------+
| 2759 |      492302 | fr       |                0 | # NOT WHAT I'M SEEING
| 2759 |      495197 | fr       |                0 | # NOT WHAT I'M SEEING
| 2759 |      538319 | fr       |                0 | # NOT WHAT I'M SEEING
| 2759 |      538823 | fr       |                0 | # NOT WHAT I'M SEEING
| 2759 |      539075 | fr       |                1 |
+------+-------------+----------+------------------+

My understanding of revision_default is the revision which is associated with the default revision of its parent (in this case, revision 539075 of paragraph 2759 should be the default revision because it is associated with the default revision (2) of its parent, node 1).

Because several revisions of the same paragraph are marked as revision_default in the database, what does revision_default mean?