I can't say exactly why this is the way it is, but in my experience there's usually a (high!) number of seats that are blocked during online check-in, even though nobody has reserved them. These have usually only become available at the airport, data points from my last couple of flights showed me, that they don't open up at any point between the start of the online check-in and a few hours before the flight. I've also observed this on A320s, so it doesn't have anything to do with equipment changes with turbo-props etc.