This is because this set is supposed to have 11 tapes and the company did not have a tape to fill the mold for the last slot. See the last photo where it shows that part 3 contains 11 tapes.
This set is Fine condition. The lamination has some rippling and staining (small dark spots). Each cover has a name written in pen on the clamshell. There is chipping on the back right plastic corner of parts 2 and 3. A cassette in part 2 has 2 X's written on it.
The cassettes are generally in Excellent condition. Comes from a smoke-free home. For more amazing treasures, updated daily!