I can not reach dice of u_net to 0.91 and std is larger than 0.06

Why can I only reach 0.81 on dice with large variance?
![1](https://user-images.githubusercontent.com/54277702/63408285-3d204280-c421-11e9-950d-40c95295f13d.png)
![2](https://user-images.githubusercontent.com/54277702/63408313-4a3d3180-c421-11e9-9824-4f8b0bcc8fc6.png)
![3](https://user-images.githubusercontent.com/54277702/63408326-51fcd600-c421-11e9-8d77-beea59b2d630.png)


