I dun think SQ will allocate seats before Checkin open. SQ does block some seats though.
That pax maybe dun like aisle or windows or maybe he/she dun like other parts of airplane. Each person have their own preference. Or maybe there is 2 paxes traveling together and since no more double seats, they just choose 2 seats which might be close. The other pax might be sitting in front or behind.
And since you purposely keep one seat, you need to expect it will be fill up esp if the flight is in Dec.