Nice to see someone else has taken interest in this. I have to say, many of the brd's I speak to don't understand this concept either and I'm always asked why I stack on the string gear for ballad.
At the moment, my string skill isn't quite capped, 335 with gear and no merits. However, I've found that this has increased the range on my ballad 2 from 15 to 18. Bearing in mind from that very handy skill table that a range of 15 with ballad 2 is 249 skill, I'm guessing ballad 3 is going to be fun to increase the range for, lol.
I think Karumac has hit the nail on the head - it was only learned at level 85, so come level 99, with more string skill available, I think we'll see a range increase. Unless of course it is a bug and nobody else has noticed or cares ; ;