Why was the Holy Roman Emperor Charles the Bald, called this nickname, when he actually had hair?

Charles the Bald (823-877 CE) was the grandson of Charlemagne and Holy Roman Emperor, reigning from 875 to 877 CE.

It has been suggested that Charles' nickname was used ironically and not descriptively, that is, he was not, in fact, bald, but rather he was extremely hairy. An alternative or additional interpretation is based on Charles' initial lack of a 'regnum', a Latin word meaning 'kingdom'. In this case, it is a tongue-in-cheek reference to his landlessness, at an age when his brothers already had been sub-kings for some years.

Contemporary depictions of his person, in his 'Bible of 845' CE, on his seal as king, as well as on his seal of 875 CE as Holy Roman Emperor, show him with a full head of hair.

In the 'Genealogy of Frankish Kings', a text from Fontanelle dating from possibly as early as 869 CE, and a text without a trace of irony, names him as 'Karol's Calvus' translated, 'Charles the Bald'. By the end of the 10th century, Richier of Reims (10th-century French monk and historian) and Adhemar of Chabannes (11th Century French monk, historian, musical composer and successful literary forger) refer to him in all seriousness as "Charles the Bald".