As Bananasx2 states the problem comes from being field limited. If you are and you call V1 early do, you enough runway to accelerate to Vr and clear the screen height in the event of an engine failure between your artificially reduced V1 and Vr? Possibly not. I think the Boeing verbiage is that a stop must be initiated by V1 but the mechanics of it allow for a certain amount of 'thinking time'. There is a value laid down but I can't lay my hands on it currently and it is therefore understood that the aircraft may accelerate beyond V1 by a certain margin before the stop is initiated. I have seen written in several manuals that a stop initiated at V1 on a limiting runway may result in the aircraft not coming to rest on the paved surface. This is due to the calculations being made for a perfect aircraft, on the perfect day, by the perfect pilot and we all know there is no such thing as the perfect aircraft or the perfect day!