This is often done when translating between languages. I did notice it a few times in the movie, when a proper noun was heard in one language but not translated in the next.
Let me explain it as I understand it:
In the common tongue (English) in which most of the dialogue takes place in this movie (and book), we have usually been referring to Erebor as "The Lonely Mountain" or simply, the Mountain. This is because "Erebor" as the Kingdom of Dwarves technically no longer exists, as it was wiped out by Smaug. The Mountain is still there; "Erebor" is not. It is like calling the city of Rome as "The seat of the Roman Empire". Rome itself is still there, the Roman Empire is not.
It seems that the term "The Lonely Mountain" has at least been around for 200 years or so, because it was on Thorin's map. Regardless, I think it is clear that this term would not make it into Black Speech - it is overly poetic and full of emotion. They would simply call it "Erebor" and would not stop calling it that just because the kingdom had fallen. It would probably not occur to the Orcs that they should start calling the place anything else. Black Speech is terse and to-the-point.
So, when the Orcs refer to the Mountain, they still call it Erebor. However, since we have been calling it "the Mountain" for the most part in the common tongue, the subtitle used that instead of Erebor. This kind of thing is done in translation all the time; if a certain term for something has been established in the target language, we would try to stay in that mode for consistency's sake, even if the language we were translating from used a different, equivalent term.