Occasionally, different screens can show different things -- especially with DVI cables and/or older DisplayPort cables -- or when a GPU doesn't support the pixel clock necessary for the overclock. Then different screens sometimes go out of sync.
Incidentially, I created an article,
Oh No, I'm At The Wrong Refresh Rate, since it was common that a monitor was not running at the correct refresh rate. Follow steps 1-5 to find out what refresh rate Windows thinks your monitor is. Get the correct number showing up there first, before using TestUFO.
There can be some disagreements between different screens, so that's the most important screen to verify the correct number shows up at.
The monitor's OSD is another method to verify correct refresh rate (at least during fixed-Hz situation).