It looks as though as the newer blocks are being added on to unconfirmed (even though many of them are mow  'confirmed') then the oldest blocks that have not been paid out are being deducted from this 'unconfirmed' total! This has been happening since block 21304 - it looks like it could take a while to properly work out rewards due to everyone. I am hoping that the due rewards will be paid automatically rather than everyone having to put a claim in for unpaid blocks 21304+ which may go unnoticed by many.