Standard pricing. Lot's of cruise lines do this. First the cruise line determines the total price for the cruise. Let's say it's $5000. They advertise 70% off the second guest which would make the second guest pay $750 (30% of $2500) but they charge the first guest $4250, total charge is still $5000.
All their discounts are a variation of this. Second guest free? The first guest pays $5000. Same for BOGO.
Completely disregard the sales promotions.
Find a cruise at a price you like and book it. If the price goes down ask for a price reduction or a FCC. If it goes up, you're golden. I've got a cruise booked for Jan 2025 that has doubled in price since I booked it.