Your "Created By" field looks like a text field instead the standard lookup.
Is possible that inside your system a custom field with label "Created By" was created, probably you can find two "Created By" inside the first dropdown (don't consider "Created By (Delegate)".
To have a confirmation just download the fetchXml of your query and check the field name inside the condition attribute, probably will not be createdby but something like new_createdby