I believe the latter is the case, just from experiments I've done over the last couple of hours. That might explain the continually-increasing count you are seeing from your testing.
Note that it is not unusual to have a few (< 10) such "slow sectors" on a drive. Personally, I don't like to see more than 10 on a new drive, especially if there are several in close proximity. Sometimes, a re-writing of the drive (or affected area) will improve the situation--the problematic sectors resulted from a "weak" writing.
Good luck (and thanks again for the enlightenment),
UhClem