The inconsistent results of the Timequest

vjAlter nailed the issue. When you double-click on Create Timing Netlist in the Task bar, it automatically creates a model in the Slow Corner, since this is what users want 98% of the time. You can go to the Netlist Pull-down menu and go to Create Timing Netlist, where a pop-up appears allowing you to choose a different timing model. Or, if you had already created a timing netlist in the slow model, you can go to Netlist -> Set Operating Conditions and change it there. If you've created some timing reports, you can right-click and re-generate them.