Generally when I've seen a category listed as only guarantee available it's a sign that they're basically sold out of cabins, but they're still taking a few more, reservations. As the sailing gets closer, it'll sort out because there'll always be some last minute cancellations, or they'll move people to other categories, or they'll start offering move over specials to people. So they're trying to make sure there's enough reservations to fill the ship, but are probably technically oversold on the cabins.

Some of those moveover specials can get pretty decent. I know I got one about 2 days before I was leaving for my 14 day South America cruise. They were offering a mini (I was booked in a balcony) on any cruise up to 30 days, with half our fare back as refundable onboard credit, plus assistance in changing airfare. And I've definitely heard of better offers, just depends on how desperate they get. It's a fine line they walk between too few bookings, a full ship, and being oversold.