Critically low on storage popup topic

I keep getting a popup that says the Fire TV is critically low on storage. Here's a df from adb shell:

root@android:/storage/emulated/legacy # df
df
Filesystem Size Used Free Blksize
/dev 973M 48K 973M 4096
/mnt/secure 973M 0K 973M 4096
/mnt/asec 973M 0K 973M 4096
/mnt/obb 973M 0K 973M 4096
/storage/usb 973M 0K 973M 4096
/system 756M 721M 35M 4096
/data 5G 2G 3G 4096
/cache 756M 13M 742M 4096
/firmware 63M 7M 56M 16384
/persist 7M 4M 3M 4096
/mnt/shell/emulated 5G 2G 3G 4096
/storage/emulated/legacy 5G 2G 3G 4096
/data/data/com.amazon.avod/files/streaming 400M 0K 400M 4096
root@android:/storage/emulated/legacy #

I don't see a problem. I'm running SPMC. That's probably the biggest space hog, but where is the problem?