It means they are holding back seats for when check-in opens. My experience from booking 10 LHR-SIN returns a year is that SQ doesn't release that many seats for advance seat reservations. If I book a month ahead, I often experience this, but if I book 3-4 months ahead I'm usually OK.

Once these are gone, there is nothing you can do as a KF elite to get your seat in advance. If you insist on flying SQ but want the aisle seat then you have to keep checking every week and cross your fingers someone who has reserved such a seat cancels.