because your hardware can't handle it, and why turn off vertical synch? it just makes it easdier on your hardware and makes your game steadier and smoother. your monitor cannot handle more than X frames/second so why would having X+10 matter if all it does is make your screen a fraction of a second off from reality while making your computer worker harder than it needs to(making it run hotter and potentially die sooner[well, not that much sooner])
unless there is a bug forcing vsynch lower than your monitors actual refresh rate, it shouldn't matter. 30 is quite low so it might be a bug on your part(which if so invalidates what I say below in your specific example) the human eye isn't capable of discernign past 60FPS or so, and if your monitor onyl redraws itself 60 times a second, and it receives 70 then 60 of the 70 will be drawn and each frame will be 1/7th off
