With an ONS, it could always be that she was just too drunk and had a lapse in judgment or something like that. But with FWB, she wants you more and makes a conscious decision without expecting demanding boyfriend stuff. It is free of responsibility. You can have the benefits without the disadvantages of boyfriend responsibilities. It is the ultimate example of how desirable you are. Or it can be that she wants more, and with FWB she thinks you are committing to her, but you still have all the power.