6 people with connections could have checked in before you.
I quite often grab a better seat when I can check in up to 24hrs earlier due to say an AMS-LHR flight on the same PNR connecting to a LAX flight later in the day.
Sometimes you get 20-40 people ahead of you, or say SEQ042 when you check in at exactly T-24.

Most of the time, should the bassinet seat be available, I'll grab that. If I stick with the aisle, you're pretty much guaranteed that someone with either grab it during OLCI or shift themselves early in the flight, and vice versa.

​​​​​​Very rare that a bassinet with 3 seats in economy doesn't get filled up. Have had the odd middle free, but generally popular.