When you mount a partition, it will show in
df -h, and if you umount it, then it will no longer show in
fdisk -l uses
/proc/partitions and prints out all partitions which are physically connected, but your USB drive is still connected to your PC. When you unplug it, then it will not show in
fdisk -l anymore, and you can also check
cat /proc/partitions for the same.
I think you are thinking from a Windows perspective, if not then let me know I will remove below content.
Let me clear these things:
df -h is the same as when you open Explorer (My Computer) and
fdisk -l is same same as the Device Manager and Disk Management.
After disconnecting(safely removed) a USB device in windows, it will still be shown in "Device Manager" under Universal USB Bus Controller