In sanity-hsm df is used to check if there is enough space on the file system before creating a rather large file (with make_custom_file_for_progress()). If there is not enough space, a call to cleanup_large_files() is issued and then df is used once again to check that there is now enough space.

The thing is, df takes time to see the update in free space, more than du for example (even with the option --sync).
To reproduce:

dd if=/dev/zero of=/mnt/lustre/file count=100 bs=1M
df -h
find /mnt/lustre -size +10M -delete && df -h [--sync ] && du -s /mnt/lustre
# Wait around 3 to 5 seconds
df -h

You can check the output of the first two df commands, the "Used" column should be the same. But du immediatly reflects the actual size.