Why does plane look empty in main cabin but really full?

Why does it look like there are so many seats available in the main cabin, but the flight ends up being full? Is it because those seats just aren’t assigned until the last minute, even though people have tickets?

Why does it look like there are so many seats available in the main cabin, but the flight ends up being full? Is it because those seats just aren’t assigned until the last minute, even though people have tickets?