"with your 10th sailing" ...not your 10th booking, seems like you will be able to use your benefits on the 11th.
It's kinda lame, it would seem like they should reward you for booking(yes technically they run the risk of you canceling). IE. If your at 8, want the benefits of 10 but will go ahead and book one for March and one for December, why not reward you already? Everyone wins!