Filesystem and gparted disagree on free memory

Question

My filesystem says I have:

While gparted says:

And if I run disk analyser on root partition only:

it mentions I only have 6Gb in total, instead of 15 from gparted.

I guess gparted is right, but I also understand I only can use the available memory seen by the system. Which means there might be some leak/miscalculation somewhere. How can I unify those? Should I do some kind of defragmentation?

Thanks

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EDIT:

Here are my terminal outputs:

▶ sudo fdisk -l
[sudo] password for augustin: 

Disk /dev/sda: 119.2 GiB, 128035676160 bytes, 250069680 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x000ecc64

Device     Boot     Start       End   Sectors  Size Id Type
/dev/sda1        62566400 209952767 147386368 70.3G 83 Linux
/dev/sda2       241467392 250068991   8601600  4.1G 82 Linux swap / Solaris
/dev/sda3  *    209952768 241467391  31514624   15G 83 Linux
/dev/sda4            2048  62566399  62564352 29.9G  5 Extended
/dev/sda5            4096  30785535  30781440 14.7G 83 Linux
/dev/sda6        30787584  62566399  31778816 15.2G 83 Linux

Partition table entries are not in disk order.

▶ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3        15G   14G  373M  98% /
udev             10M     0   10M   0% /dev
tmpfs           774M  9.7M  764M   2% /run
tmpfs           1.9G  195M  1.7G  11% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/sda6        15G   11G  3.1G  79% /media/partition2
/dev/sda5        15G  7.7G  6.0G  57% /media/partition1
/dev/sda1        70G   56G  9.6G  86% /home
tmpfs           387M   36K  387M   1% /run/user/1000

And my full gparted, to make it clearer:

So my /home repository is not in the same partition as my /.

Also I use Docker, which might have something to do with memory usage I think.