Well, one reason would be the grand split between the sons - first time we've documents in medieval French and German as well as Latin - the aftermath of Alexander effect, in other words. All the fighting. And Ludwig, who was Emperor after Carolus, was deposed by his own sons twice.

Theophanu: Here she is with her husband (http://de.wikipedia.org/w/index.php?title=Datei:Otton_II_et_Th%C3%A9ophano.JPG&filetimestamp=20060206041356), and notice they're both depicted the same size.

The wedding contract (http://upload.wikimedia.org/wikipedia/commons/c/cb/Urkunde01.jpg), which mentions she was crowned as Empress in Rome by the pope. She scandalized the court somewhat by bathing once a day, and also because she used a fork, but most of all because she signed her documents as Emperor, as had been the case with earlier Byzantine empresses ruling for their sons in Byzantium, but most certainly nowhere in the German speaking territories. So she was Theophanius gratia divina imperator augustus and is counted as a ruler among the list of German emperors.