Why does 'df' omit some NFS shares in its output on Red Hat Enterprise Linux 7?

Issue

• Why does df omit some NFS shares in its output on Red Hat Enterprise Linux 7?
• When issuing the df command some NFS shares are not visible in the output.
• df -h does not show autofs direct mounts
• When the command is issued with the -a flag the shares are visible.
• The NFS shares are being shared from the same server under the same directory tree, for example:

server.example.com:/share
server.example.com:/share/foo
server.example.com:/share/bar