Because you see faster than you can react. Response time has to do with ghosting, how fast the pixels change color. You could consciously notice a flash of light that is 4 ms. Remember the spec is best time, average will be higher. Usually one stating 5ms shouldn't get too high on average and have ghosting issues but you should always check reviews. Tftcentral is the best monitor reviews but they can't possible have all models so most of the time you just have to go by what other users say.