At first I thought it was because the Quantum cruise was under 10 days, but now I see it is an 11 day cruise so it should have the same booking bonus as your Explorer cruise. I don't know the answer. Perhaps they have different rules for the Quantum? Could be just an error on the reservation? You'll probably have to call them or your agent to find out.
I vaguely recall some asterisked note about Quantum bookings on the flyers they had at the loyalty desk on the Serenade Of The Seas in January, but I don't have a copy of the form.