This problem may occur in the following cases
The workflow is configured to start on item created, in this case, once you add a new item the
Modified By = Created By.
So make sure that it also started on the
You may be trying to edit the item with the same created user, so the result would be
Modified By = Created By too.
There is a problem that has been resolved in DEC 2013 Cumulative Update that closes to the description you mentioned
When you create a list workflow in which platform type is SharePoint
2013 workflow in SharePoint Designer 2013, the workflow may set an
incorrect user name in some list item fields.
Check the details at Description of the SharePoint Server 2013 hotfix package (Sts-x-none.msp): December 10, 2013