"docker system df" doesn't add up to "docker info" data space

Docker Engine General
1

Hi. I’m encountering a storage shortage problem of my server.
I wanted to know which container using most storage space, so I ran “docker system df” command(and, “docker system df -f”),

# docker system df
TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              132                 39                  28.38GB             21.17GB (74%)
Containers          81                  72                  240.2MB             4.498kB (0%)
Local Volumes       9                   5                   135.9MB             67.97MB (50%)
Build Cache                                                 0B                  0B

But, I noticed that the size(including with -f option) doesn’t add up to the output of docker info command:

# docker info
Containers: 81
 Running: 72
 Paused: 0
 Stopped: 9
Images: 132
Server Version: 17.12.1-ce
Storage Driver: devicemapper
 Pool Name: docker-thinpool
 Pool Blocksize: 524.3kB
 Base Device Size: 10.74GB
 Backing Filesystem: xfs
 Udev Sync Supported: true
 Data Space Used: 60.3GB
 Data Space Total: 81.6GB
 Data Space Available: 21.3GB
 Metadata Space Used: 18.91MB
 Metadata Space Total: 855.6MB
 Metadata Space Available: 836.7MB
 Thin Pool Minimum Free Space: 8.16GB
 Deferred Removal Enabled: true
 Deferred Deletion Enabled: true
 Deferred Deleted Device Count: 0
 Library Version: 1.02.140-RHEL7 (2017-05-03)

Where I can find hidden used spaces and reclaim them?