du Weirdness - how is this possible

[Philip Rhoades]

People,

I started deleting GBs of stuff from:

/dev/sdb1 /backup

but df did not reduce from 95% so I looked more closely and found this 
weirdness:

# du -s -BG 20180216
43G     20180216

# du -s -BG 20180216/*
1G      20180216/naf_dirs
43G     20180216/phil
1G      20180216/root

# du -s -BG 20180216/phil/*
1G      20180216/phil/0
1G      20180216/phil/0_finance
1G      20180216/phil/0_naf
1G      20180216/phil/Maildir
1G      20180216/phil/txts
1G      20180216/phil/vimwiki

Where has ~37GB disappeared to?  There are no files held open and I have 
successfully umounted and re-mounted the partition - what is going on?

Thanks,

Phil.