If you are talking about same day type of flight load changes, then it is all down the the airline's Same Day Confirmed (also called alternate flight same day) policy. It can be very inexpensive to make same day changes so flight loads, which used to be fairly stable the day before travel are now all over the map as people adjust their flights for anywhere from $25-75. Most airlines allow you to either call, go online or simply show up at the airport and change to an alternate flight for that price range which means nonrevving becomes infinitely more fun - we have no idea what the flight loads will actually end up being until we show up!

An example: I was coming home from California yesterday and the original flight I was going for looked bad so I checked out an alternate airport. Loads of seats about 8 hours before so I modified my plans. On the way to the airport I checked again and my "loads of seats" became 0 seats with 42 - yes 42 - people listed. I ended up doing something I have never had to do when flying nonrev - I bought a ticket on the way to the airport (thank goodness for mobile apps). In the end 5 or so of those 42 people got on and I definitely wouldn't have been one of them.