That's a function of your monitor.
It increases respond speed for some small artifacts.
I noticed the same when I had my Samsung monitor set to RTA.
You can turn RTA (response time acceleration) off by going into the menu of your monitor and disabling it.
PS If you record it with something like FRAPS it won't show up at all, because your monitor applies it. (after it is processed by the GPU)
A bit sad that when respnse time is set to normal the picture is actually quite washed, a bit blurry and not so responsive :<
But thanks anyway +rep