Low Validation Loss but IoU metric is small also

Hi all
i am working on semantic segmentation and the loss is Focal loss
and when I try to get the validation loss and changing the Gamma value of focal loss to get another loss, I found two cases:

     case1: small validation loss with small IoU metric
     caes2: higher validation loss with higher IoU metric 

Example: for validation loss 0.07 the iou=80% but for validation loss 0.01 the iou=70%
Why did this happen? I am confused