It looks like you paid in advance for a choice of seat. Most people on that flight obviously didn't, instead they waited until check in and selected their seat for free or were allocated a seat when they checked in. It doesn't mean they didn't already have tickets well in advance, it just means they had not paid extra for a specific seat selection. For those who do not pre pay for a seat then the remaining seats are taken on a first come first serve basis at online check in.