الْخَتْمُ مُخْتَلِفٌ. الْخَتْمُ غَيْرُ مُوَافِقٍ.
The seal differs. The seal does not correspond.
If the second clause reinforces substantially the same judgment, the relation can approach complete connection by emphasis. By contrast, al-khatmu mukhtalifun. Fa-lā najzimu bi-l-nisba adds an inference: the seal differs, so we do not assert the attribution conclusively. That is not merely a synonymous restatement.
An analyst must ask whether the second clause reasserts the earlier proposition or answers a further question. Proximity and similar vocabulary cannot decide the rhetorical classification by themselves.