The last update of Pinball Arcade on iOS was accompanied by a renaming of the app. Pinball arcade is now known as Pinball Arcade Plus. The new version, 6.8.5, is supposed to bring updated tuning, a plunger button, updated sounds and some fixes.
I don't know what spurred the renaming of the app. My guess is that since recently we have seen several iOS pinball games incorporating the moniker "arcade" into their title or their description, FarSight opted for the "plus" qualifier in order to distinguish their app form the intruders. A simpler explanation could be that, since there exists a free version of the Pinball Arcade, FarSight decided to make a clear distinction by renaming the paying version as "plus".
As to what are the real improvements in this new version, I guess we'll have to wait and see.