If you had to purchase tires for your car to get to work and they offered to sell it to you for $800, you wouldn't say "thats nice, but i'd rather pay you $1,500 instead because i'm nice and i want to help you out"

would you?

Neither would United. Normally its not a problem. They make decisions that are best for them and acceptable to their customers. They offer VDB's not only because they are required but because they are financially less burdensome on them than IDB's. They increment the VDB offer until the point it is no longer better financially for them then an IDB. When it reaches that point, if they still need volunteers, they do, as the CoC permits, an IDB.

its really as simple concept. Just so happens, on this one occasion, something unpredictable occurred and their total value was reduced.