disk space utilization - df on 7.7GB shows 400MB available while du -sh / reports 3.8GB used

I believe df is reporting incorrect disk usage, as I have the following problem:

I deleted a few files to free up space, then run the following:

$ df -H

reports 7.7GB on Volume and 400MB available, however, running:

$ du -sh /

reports 3.8GB usage, on this volume. There is a loss of roughly 3.9GB.

I have ensured there are no opens files using $ lsof and also rebooted the server after deletion.

FYI - OS Version: Centos 6.5

I would appreciate it if you would be able to guide me or point me in the right direction. Thank you.