Why BlockIOWriteBandwidth is not limiting the write bandwidth for a specified device in Red Hat Enterprise Linux 7 ?

Issue

• Why BlockIOWriteBandwidth is not limiting the write bandwidth for a specified device in Red Hat Enterprise Linux 7 ?
• BlockIOWriteBandwidth restriction set to 5 MB/s but iotop shows 100 MB/s instead of 5 MB/s. Similar behavior is also noticed while starting the dd using systemd-run and then setting the property using systemctl set-property command. Sample systemd unit configuration file

/etc/systemd/system/mybackup.service
[Unit]
Description=mybackup
[Service]
CPUShares=10
BlockIOWeight=10
BlockIOAccounting=1
BlockIOWriteBandwidth=/dev/mapper/rhel-root 5M
ExecStart=/usr/bin/dd if=/dev/zero of=/backupfile bs=1024 count=10000000
Restart=always
[Install]
WantedBy=multi-user.target