Why do some PostHistory 'partner' events not match up?

Question

There are some events that get logged in the PostHistory database that, as far as I can tell, always happen together:

• a post gets merged: PostMergeSource, PostMergeDestination
• a post is created: InitialBody, InitialTitle, InitialTags
• similar for rollbacks: RollbackBody, RollbackTitle, RollbackTags

So why, when running a query over Worldbuilding's database, do I get these results?

Site            History Type Name       Instances
-------------------------------------------------
worldbuilding   Initial Body            25658
worldbuilding   Initial Title           4391
worldbuilding   Initial Tags            4391
worldbuilding   Rollback Body           49
worldbuilding   Rollback Title          23
worldbuilding   Rollback Tags           14
worldbuilding   Post Merge Destination  7
worldbuilding   Post Merge Source       1

It would seem to make sense for the numbers for each partner event to be the same, yet they're not. Why is this? Am I misinterpreting the events?

---

_{Data from the data dump imported into a MySQL database programmatically. Query:}

SELECT
    `ph`.`Site` AS 'Site',
    `pht`.`Name` AS 'Type Name',
    COUNT(`ph`.`PostHistoryTypeId`) AS 'Instances'
FROM `PostHistory` AS `ph`
INNER JOIN `PostHistoryTypes` AS `pht` ON `ph`.`PostHistoryTypeId` = `pht`.`Id`
GROUP BY `ph`.`PostHistoryTypeId`, `ph`.`Site`
ORDER BY COUNT(`ph`.`PostHistoryTypeId`) DESC
LIMIT 1000;

Answer 1

Answers do not have titles or tags, so InitialBody is expected to be larger, while InitialTitle and InitialTags are equal.

Rollback* revisions are dependent on what fields had modifications, so you can have 1 - 3 (body, title, and tags) corresponding events for the same rollback for questions, and body rollbacks for answers.

I'm not positive about the merge information though, I'm looking into that. Records corresponding to deleted content are typically removed from the data, so while you'll typically always have PostMergeDestination records, PostMergeSource records might be harder to come by.

In the case of Worldbuilding, the single PostMergeSource points to a non-deleted merge stub which might be behaving differently because it was migrated, closed, and then locked before being merged.