Windows' beautiful curse.
Mouse Negative Acceleration comes free with every copy of windows and without you knowing, hurts your aim. Let's say right now you "know your mouse" only because you dont know any different. Well when you snap your mouse real quick there is negative acceleration. Ever try to turn real fast because someone jumped out in front of you and your mouse wasn't "there" for you but your muscle memory knows you made the correct movement? That was negative acceleration. You moved so fast that it under-performed. Removing the negative acceleration will allow a True 1-to-1 movement ratio like you Thought you had.
Thankfully several people over the years have created fixes such as Cheese fix and CPL mouse fix. MarkC has compiled everything into a nice package and has even gone ahead and included windows 8.
*NOTE* 1000hz on an AMD Phenom x4 965 I can pull 8% CPU usage with my mouse. Assuming modern CPU's can handle any refresh rate is a myth, test for yourself, lag spiking during a firefight is not a good idea.