The report writer also failed to realize they'd looped back to their starting location until much after they had done so. They're clearly unreliable, an inconsistency like that doesn't need to be explained by anything other than that the author is losing their grip on reality. The other explanation is that their instruments are indicating a diameter of 15,000 light years and that that is all they are recording (as they were recording in previous reports, the numbers came from instruments), and the report writer has failed to recognize the inconsistency between their belief and the facts available to them.