Currently, the VersionHistoryIndex doesn't take into account versions that were introduced via Relationships or TopicReferences. Ideally, those should be factored in, due to the scenario where there isn't an update to the Attributes or ExtendedAttributes.

In practice, this should be rare, since the OnTopic Editor will, by default, write an indexed LastModified and LastModifiedBy attribute for every save. If the change is done programmatically, or the LastModified attributes are disabled, however, we should ideally make sure all versions are still accounted for, even if no attributes were modified.