Iou Metric formula

Why does Fastai use
union-intersect compared to just union only that we find in IOU formula

def dice(input:Tensor, targs:Tensor, iou:bool=True)->Rank0Tensor:
    "Dice coefficient metric for binary target. If iou=True, returns iou metric, classic for segmentation problems."
    n = targs.shape[0]
    input = input.argmax(dim=1).view(n,-1)
    targs = targs.view(n,-1)
    intersect = (input*targs).sum().float()
    union = (input+targs).sum().float()
    if not iou: return 2. * intersect / union
    else: return intersect / (union-intersect+1.0)