The high flight number indicates an extra flight that is not a part of the regular schedule. In this case they need that aircraft there in ORD for a charter for a professional sports team. Rather than ferry the aircraft there without passengers or take some other regularly scheduled flight to Chicago and have it as the charter they bring an extra flight up to Chicago so they don't mess up the rest of the schedule. For the aircraft returning to Atlanta after its charter operation see flight 9675 on 10JUN.
Other times you will see an extremely high flight number are at the last minute, for example if there is an extremely high load factor in a particular market for whatever reason you may see an extra flight with a 9000 series flight number. Also if there is an extreme delay on a through flight it may be split into two flights the latter of which will have a flight number in the 9000s. Charter flights themselves have 9000 series flight numbers, but they will not be available for sale to the general public. Hope this helps