The update site in Joomla's database does not match the update site advertised in the extension's XML Manifest. You might not be receiving updates for this extension, or only seeing updates to its older versions. This can also cause this extension to appear as “incompatible” when updating Joomla!.
The extension's XML manifest contains the definitions of the update site(s) known to this extension. However, Joomla's database (the #__update_sites table) shows a completely different update site URL for this extension.
This may result in inability to update the extension, or the extension showing as “Incompatible” when updating Joomla to a new minor or major version.
Onthos lets you ask Joomla! to rebuild the updates sites. This tells Joomla to go through the manifests of all installed extensions and install any missing update sites into Joomla's database.
This can and does happen a lot in Joomla. When you are updating an extension and its new version defines an update site different to the one from its previous version, you'd reasonably expect Joomla! to change the update site to the new definition. Oh, my sweet summer child…
Joomla will try to create a new update site with the new update site information, without touching the old update site. Sometimes, however, it will fail to do that and just keep the old update site. Both behaviours are nonsensical which leads to this problem cropping up a lot despite developers' attempts to work around it.