I can see why they would do this, because Wondrous Tails has two entirely different kinds of "books" you can get from Khloe depending on your level.
- If you don't have a max-level character, then every duty in your book will be level 60 or below.
- If you have a max-level character, then many of your duties will have significantly higher levels.
Naturally, the book that caps out at 60 is incredibly easy to finish; a level 90 character can solo the whole thing in 20-30 minutes.
But the max-level book will generally require you to actually enter duties with other players, and by playing with other players and adding your body to higher level content, you help sustain the multiplayer aspect of the MMO.
So to me, it makes perfect sense that these two books should have different rewards.
That said, I do wish it was easier to realize that there were these 2 different kinds of books. I've played for years and never noticed this until a friend told me about it and demonstrated the difference. If they communicated the difference better, it would reduce the likelihood of someone feeling disappointed like OP.
For what it's worth, I'm really sorry for the disappointment OP. I can imagine it feels like a punch to the gut thinking you'd wont the big prize only to realize you hadn't.