When you calculate V1 you are using an unreliable fudge to find the point on a balanced field runway from which you may stop or accelerate to Vr before the end.
I say 'fudge' because, ideally, you'd have a mark on the runway as I seem to recollect was used by the V Force.
The use of IAS doesn't take into account an unnoticed power shortfall or a dragging brake which will place your V1 point too far down the runway.
Like Vmcg and Vmca there are woolly areas but they are the best we have at present.