Each route is carefully monitored. ORD being somewhat business heavy will experience a a greater degree of passengers changing travel plans at the last minute. Ticket costs will also reflect this. By comparison a leisure heavy route like Mauritius will experience far fewer passengers changing plans who will generally be on cheaper more restricted tickets. Over a period of time, data will be calculated to an algorithm that sets a tolerance to overselling. And airlines are allowed to do this. A route like ORD-LHR will generally experience a higher oversell than a primary leisure route like MRU-LGW, however things such as school holidays can change the dynamic somewhat and there will be times where more passengers turn up than there are seats. I guess on days like this BA are going to also hope that there will be a certain amount of connections from AA that for whatever reason miss the cutoff for check-in and aren't able to fly. I imagine a lot of the passengers affected (such as the guy checking in next to your wife) will be cleared at a later stage once check in has closed.