Filesystem not recognizing/using all available space in partition

Question

I am running dual boot Ubuntu 18.04 with Windows 10. I installed Ubuntu as an afterthought, and originally split the disk roughly down the middle between the two. I later reduced the size of the Windows 10 partition and moved the Ubuntu partition to take over the new free space. Unfortunately, Ubuntu doesn't seem to recognize all of its new free space--it is using some, but not all, of the newly acquired space.

lsblk reports:

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
loop0    7:0    0  89.1M  1 loop /snap/core/8268
loop1    7:1    0  14.8M  1 loop /snap/gnome-characters/495
loop2    7:2    0  44.9M  1 loop /snap/gtk-common-themes/1440
loop3    7:3    0   3.7M  1 loop /snap/gnome-system-monitor/127
loop4    7:4    0  14.8M  1 loop /snap/gnome-characters/399
loop5    7:5    0   956K  1 loop /snap/gnome-logs/93
loop6    7:6    0   956K  1 loop /snap/gnome-logs/81
loop7    7:7    0   4.3M  1 loop /snap/gnome-calculator/704
loop8    7:8    0 160.2M  1 loop /snap/gnome-3-28-1804/116
loop9    7:9    0   3.7M  1 loop /snap/gnome-system-monitor/135
loop10   7:10   0   4.2M  1 loop /snap/gnome-calculator/544
loop11   7:11   0  54.7M  1 loop /snap/core18/1668
loop12   7:12   0  91.4M  1 loop /snap/core/8689
sda      8:0    0   477G  0 disk 
├─sda1   8:1    0   579M  0 part 
├─sda2   8:2    0 174.8G  0 part /home/user/Windows
└─sda5   8:5    0 301.6G  0 part / 

GParted agrees: disk size: 301.57 GiB; Used: 273.14 GiB; Unused: 28.43 GiB

But df -h disagrees.

Filesystem      Size  Used Avail Use% Mounted on
udev            7.8G     0  7.8G   0% /dev
tmpfs           1.6G  2.0M  1.6G   1% /run
/dev/sda5       296G  268G   14G  96% /
tmpfs           7.8G   88M  7.7G   2% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           7.8G     0  7.8G   0% /sys/fs/cgroup
/dev/loop2       45M   45M     0 100% /snap/gtk-common-themes/1440
/dev/loop0       90M   90M     0 100% /snap/core/8268
/dev/loop3      3.8M  3.8M     0 100% /snap/gnome-system-monitor/127
/dev/loop4       15M   15M     0 100% /snap/gnome-characters/399
/dev/loop1       15M   15M     0 100% /snap/gnome-characters/495
/dev/loop5      1.0M  1.0M     0 100% /snap/gnome-logs/93
/dev/loop7      4.4M  4.4M     0 100% /snap/gnome-calculator/704
/dev/loop6      1.0M  1.0M     0 100% /snap/gnome-logs/81
/dev/loop8      161M  161M     0 100% /snap/gnome-3-28-1804/116
/dev/loop9      3.8M  3.8M     0 100% /snap/gnome-system-monitor/135
/dev/loop11      55M   55M     0 100% /snap/core18/1668
/dev/loop10     4.3M  4.3M     0 100% /snap/gnome-calculator/544
/dev/loop12      92M   92M     0 100% /snap/core/8689
/dev/sda2       175G  -29G  204G    - /home/user/Windows
tmpfs           1.6G   16K  1.6G   1% /run/user/121
tmpfs           1.6G   40K  1.6G   1% /run/user/1000

I can account for the ~5G difference between disk size in df vs lsblk/gparted with swap:

$ du -h /swap*
2.1G    /swapfile
1.1G    /swapfile2
1.1G    /swapfile3
1.1G    /swapfile4

I know that I'm not going to get everything out of my disk/partition, but a 14 GiB difference in free space is make-or-break for a couple extra VMs. Any thoughts?