We were on the same cruise. We got an additional 9pm time in Toscana on departure day and arrived 20 mins early and were seated immediately. We got a call a few days into the cruise asking us if we wanted to move our 2 pre-booked 9pm reservations for later in the cruise to 8pm. It did seem they held a lot back just in case of VIP situations, and are probably overly cautious when calculating dining times. I guess they want to be 100% sure guests arriving for the later slots wouldn't have to wait, rather than deal with that backlash.
We actually canceled the second Toscana visit because the first time wasn't that great for us personally. We still received the reservation reminder on the cabin door the next day, so I went to the desk again to confirm our reservation was removed from the system, it was, however neither time did they ask, even out of curiosity why we were canceling and not attempting to book another date/time. That felt odd.