This might sound....noobie

How is it when I have a timer set at 1000 ticks a second it will achieve this rate when vsync is off on dxdraw but when vsync is on, the interval is still set to 1000 but runs at the refrsh rate (in my case 75 a second)?

I thought the screen was refreshed at this rate, why is the timer itself firing at different speeds? How can dxdraw/refresh rate have an effect on the timing of the timer/application?