A question about the ablation study in the paper

[YaN9-Y]

The authors mentioned that the refined stage of the Coarse-Refined network has blurred the image in Chapter 4.1 of the paper, and they gave the reason as

'We discovered that the reason for this is because L1 loss about output of refined network is always smaller than of coarse network'.

I'm not quite understand it. I would really appreciate it if someone could give me a more detailed explanation.