A very simple and straightfoward explanation: Transcription error. The monks of the Roman Catholic church badly mastered the Cyrillic script. So one of them, when writing a document, took Серб for Серв. For those who write both scripts it is I guess a well-known phenomenon that one mixes both scripts when writing in a hurry. Serbs always stayed alien to the axis powers of Europe, even though they enjoyed fargoing priviliges from the Kaiser auf Austria when protecting the Militärgrenze (Kraina) against the Turcs. They were in fact free – own land, no taxes (like Kossacks in Russia). The more interesting seems the fact that for the Roman Catholic world they were branded as “unfree”.