Fix Week
May be the Pethouse is sold only as fixed week? Fixed week units are automatically reserved for the owners. The only time you will see them is when the owners cancel their reservation. Then the thought is that HGVC may use it to rent out instead of opening it for club booking.
Another thought is that the Penthouse is sold as a Individual unit for the whole year? I know when the rooms at Trump building went on sale, the penthouse were the first to go at about $1.5 million a unit, plus or minus. Penthouse units sold out the first day.
A more simple explanation is that the penthouse units are not ready for use. HGVC Waikoloa is suppose to have 3 bedrooms but they do not show up on the reservation system yet. Most likely reason is because they are not built yet.