Why is Borg a 'Jarl', while everyone else is an 'Earl'?

I believe this is to emphasize the fact that he is foreign/distinct from the other viking Earls. The character bio on the History Channel site states:

cunning and devious leader of Gotaland .... refuses to cede any of his land to King Horik, whom he considers to be a foreign king with no legitimate claim

emphasis mine above.

The foreign aspect is also supported on Vikings Wikia:

Jarl Borg was originally supposed to be a Swedish Jarl, living in Sweden. But Swedish actor Gustaf Skarsgård, who plays Floki, told Micheal Hirst that "Sweden" did not exist as such in the time of the Vikings and that Uppsala would be located within it if it did. Hirst then changed Borg to a Geatish jarl, since there would not be any motivation for Ragnar going home to Kattegat before visiting Borg if he was already in Svealand (Land of the Swedes).

Small but simple difference. An earl would control 1 area or tribe, and likely pay homage to a king with several earls. A jarl is a man who may not quite be a king, but may rule several tribes and even have earls under him, but A Jarls territory is likely to be OUTSIDE and seperate of any kings territory, where as an earl is likely to be under a kings rule and territory

Jarldom has been described as being similar to that of Dukedom.

Earl is equivalent to a Count if you try to compare early feudal societies with societies such as the Norse.

In relation to the series, Gotaland should be an island in the body of water adjacent to the southern coast of Sweden. It would make sense that Jarl Borg was much harder to subdue that other landlocked Earldom, and also that he rules over all of the Cheif's on his island.

Norse society was a very complicated, the show has gone to great lengths to keep it simple for us whilst still keeping it authentic.

The OP is wrong about it being proper to use the title earl in Vikings merely because the show is made in England. Nobody ever used the title of Earl except in England and by appointment by an English king.

Everybody knows vaguely that a jarl is sort of a Scandinavian nobleman, so there is no need to translate jarl into earl so English audiences will have some idea of what a jarl is, while American, Canadian, Australian, etc. English speaking audiences will not have so much of an idea anyway.

Everyone knows that an English earl is sort of equal to a continental count, so it would not be necessary for English audiences to describe a continental nobleman as a French vice earl, or a princely earl, or a land earl of Hesse, or a border earl of Brandenburg, or a city earl of Nuremburg, or an earl palatine of the Rhine.

It would also be fairly accurate to translate jarl into English as lord.

I think i got an answer, Earl is name for nobble man inside the land of the King, Emperor, Despot, Knyaz, Furst etc. He control only a city and village around the city. His title is like a Count. Jarl is controller of a Province. He is independent from any King but not powerful as the King. His title is something like Duke or Furst.

Norse order was not like in other parts of Europe in other parts of Europe Earl must pay to the King tax but Earl in Norse lads must't(so he was independent on some level). But he will still obey a word of a King like Ragnar did with Horik.

It's a large topic it need time to be explain but this is in short. :D

Jarl literally means chieftain so there were lots of them throughout the Viking world all you have to do is read history. The only case I can see as to the reason why they call them Earl's instead is so some can understand

Purely educational, and - perhaps - culturally reverent reasons, I think, like when they occasionally (try to) speak norse. The reverence, however, stops with those tidbits of penance..