output of cat /proc/sys/fs/inode-nr vs df -i

What’s the difference between the output of

cat /proc/sys/fs/inode-nr 

and

df -i 

Both are documented at returning the number of inodes in use, but they return apparently unrelated numbers.

$   df -i Filesystem      Inodes  IUsed   IFree IUse% Mounted on devtmpfs        954448    464  953984    1% /dev tmpfs           956943      1  956942    1% /dev/shm /dev/xvda1     1310720 272395 1038325   21% / /dev/xvdb1     1966080  26278 1939802    2% /tmp tmpfs           956943      1  956942    1% /hdd1/mnt/data $   cat /proc/sys/fs/inode-nr 199563      122925