Why did Servia become Serbia?

Reading contemporary histories of the First World War, I noticed that at the start the nation in the Balkans is referred to as Servia, but in numbers published after the back half of 1916, it has turned into Serbia.

Is there any particular reason for this?

Consulting Google NGrams gives:-

which seems to correspond to my observation.

Edit Replaced Google NGram with version using capitalised names, as suggested by Steven Littman; the result makes rather more sense.

From a not credible source:

In Ancient Greek, the letter Beta was pronounced like English B. But in Modern Greek, it's pronounced like English V. So "Serbia" became "Servia."

and from another similar source :

It is called Serbia using the Cyrillic script and Servia when using the Latin script.

and finally a credible one could be find at http://paperspast.natlib.govt.nz/cgi-bin/paperspast?a=d&d=NOT19150305.2.51

Servia was the medieval Latin spelling of Serbia according to Etynomline.

Servia:

• Historical English term, taken from Greek language, used in relation with Serbia, Serbs or the Serbian language. Wikipedia

Serb:

• 1813, but in reference to the Wends; 1861 as "native of Serbia," from Serbian Srb, perhaps from a root meaning "man." Serbian is attested from 1848 as a noun, 1876 as an adjective. More common in 19c. was Servian.

Servian:

• 754 (n.), 1723 (adj.), from Medieval Latin Servia, from Serb Serb (see Serb).

Etymonline

Servia suggested a false connection with Latin servus 'slave', reflected in English serf and servile (whereas serve and service have more positive denotations). Similarly, Rumânia was changed to România to reflect Romanian român 'Roman' rather than rumân 'peasant, serf', although too late to affect French Roumanie (English has mostly switched from Roumania to Romania). In both cases the desire was to avoid a name that sounded like "Serf-land".

Actually the connection with Latin "servus" is from the correct medieval title of the numerous Balkan vassal statelets (Bosnia, Duklya, Zeta, Rashka...) which were generally named by the Roman Catholic Curia as "Regni Serviae" - 'Kingdoms at Service':