Reclaimable image size can not be reclaimed after "docker system prune -a"

Description

The size under RECLAIMABLE column has not been reclaimed by docker system prune -a.
The size was not changed, even though no size is reclaimed. How to be calculated the RECALIMABLE size ? How to make the reclaimable size to 0% ?

And is docker system prune -a safe for running containers ?

# docker system df
TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              22                  21                  6.665 GB            1.642 GB (24%)
Containers          54                  53                  301.2 MB            0 B (0%)
Local Volumes       2                   2                   1.459 GB            0 B (0%)

# docker system prune 
WARNING! This will remove:
    - all stopped containers
    - all volumes not used by at least one container
    - all networks not used by at least one container
    - all dangling images
Are you sure you want to continue? [y/N] y
Deleted Containers:
5598c73d725e4236d4df45cda6d4e75d444e51b0f72c447bfce5c793f9712345

Deleted Volumes:
aa8c141f6efa09475c93c20e8c45005bac8f8d643eed5c404cdd142e998dd329

Total reclaimed space: 0 B

# docker system prune -a
WARNING! This will remove:
    - all stopped containers
    - all volumes not used by at least one container
    - all networks not used by at least one container
    - all images without at least one container associated to them
Are you sure you want to continue? [y/N] y
Total reclaimed space: 0 B

Describe the results you received:
Not to be 0% of the reclaimable size, even though docker system prune can not remove unused images any more.

Describe the results you expected:
Reclaimable size would be 0% after docker system prune.

Output of docker version:

# docker version
Client:
 Version:         1.13.1
 API version:     1.26
 Package version: docker-1.13.1-88.git07f3374.el7.x86_64
 Go version:      go1.10.2
 Git commit:      07f3374/1.13.1
 Built:           Thu Dec  6 07:01:49 2018
 OS/Arch:         linux/amd64

Server:
 Version:         1.13.1
 API version:     1.26 (minimum version 1.12)
 Package version: docker-1.13.1-88.git07f3374.el7.x86_64
 Go version:      go1.10.2
 Git commit:      07f3374/1.13.1
 Built:           Thu Dec  6 07:01:49 2018
 OS/Arch:         linux/amd64
 Experimental:    false

Start by reproducing the reported output with docker system df, docker system prune, and docker system prune -a on Docker 1.13.1, then compare the reclaimable calculation with the prune results. Check the CLI entry points for these commands and use the issue's running-container question as a safety case. Done means the reclaimable value and prune behavior are consistent, or the expected behavior is clearly documented.