Well, if there's only one seat left for sale in each cabin, that sounds like a pretty full flight. There are typically a number of seats in Y that are blocked near the front and rear for special needs pax, UAMs, and such and not released until later. There can be similar seats in F and C+. On a 757, these blocked seats would be 17BC, 18DE, 19BC, 21DE, and rows 44 and 45 in the back.