When you delete something on Lemmy, by default, it just hides it from the public. This is so it can be restored later if deleting was a mistake. This means that it's still in the database even if you "delete" it.

In order to fully delete a post you need to purge it instead, which is an option in the ellipsis ... menu when the post is in full page view. If you wish to purge a post from the database entirely, and for whatever reason you don't have the ability to, please contact a mod.