There's a couple of ways to explain that. First is that Bioware just conflated a proper title with a generic descriptor; indeed, you can see evidence of this from the game's initial release when the original PvP Valor system was more recognisable -- one of the PvP titles anyone could get was "battlemaster." This isn't anything new, either -- for example, there is a difference between a military captain and a ship's captain, and navies real and fictional usually have to go to great lengths to disambiguate these two different senses of the word captain (which gives us terms like skipper, master and commander, post captain and so on). My point being that there is clearly a difference between a Jedi Battlemaster and a "battlemaster," in the same way that Lieutenant Commander Joe can be captain of Boaty McBoatface, even though he's not a captain.
The other point here is that it's established by The Jedi Path that the New Sith Wars era's decentralised Jedi are littered with self-styled Jedi Battlemasters, in contrast with the more formal structure of the Jedi Order usually only having a single Battlemaster at any one time. I suppose one could make an argument that something similar is in play in the current setting of SWTOR, since Bioware has rarely given us much detail about the exact structure and organisation of the Jedi in this time period.
edit ~ though it should be noted that it is established that SWTOR's Jedi Order maintains a single Battlemaster. Kao Cen Darach and Jun Seros are clearly listed as such, and Satele Shan promotes the player to the post in such a way that it makes it clear that it's a one-person job.
