Isn't getting 170GB on an LTO2 actual possible, I mean, I know it looks low, but it is entirely possible that the limit of the tape was reached. LTO2 theoretical max is 200GB to 400GB. If the data stored is not compressible, then you'll get around 200GB on the media, but only if it was streaming. If the drive wasn't fed quickly enough you'll have zillions of IRG (inter record gaps) in the tape where the tape had to stop, rewind and re-position to a point just after the last write - so, it is quite feasible that you only have 170 GB on a volume.
Is there just one tape with this symptom? Or many?
What the maximum on any one of the other tapes of the same brand/batch/set?
I have LTO1. My average in one library is 180GB, and the average in the other library is 205GB/media (out of 600 media in each library) - some of my LTO1 media have over 500GB on them, therefore I must have several with much less.