This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.
From some comments in the files /etc/bash_completion.d/mount:
# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#
# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#
Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:
# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#