Why did the mac address of bond changed on CentOS7 after reboot

I have two interfaces bonded together in LACP mode (mode=4 802.3ad) in bond0 : The mac address of bond0before reboot :

$ cat /sys/class/net/bond0/address
xx:xx:xx:xx:xx:bf

The mac address of bond0after reboot :

$ cat /sys/class/net/bond0/address
xx:xx:xx:xx:xx:bd

Here is some information about the network configuration :

$ ip -o l | grep state.UP
2: em1:  mtu 9000 qdisc mq master bond0 state UP mode DEFAULT group default qlen 1000\    link/ether xx:xx:xx:xx:xx:bd brd ff:ff:ff:ff:ff:ff
3: em2:  mtu 9000 qdisc mq master bond0 state UP mode DEFAULT group default qlen 1000\    link/ether xx:xx:xx:xx:xx:bd brd ff:ff:ff:ff:ff:ff
7: bond0:  mtu 9000 qdisc noqueue state UP mode DEFAULT group default qlen 1000\    link/ether xx:xx:xx:xx:xx:bd brd ff:ff:ff:ff:ff:ff
$ grep BONDING_OPTS= /etc/sysconfig/network-scripts/ifcfg-bond0
BONDING_OPTS="mode=4 miimon=100 lacp_rate=1"
$ egrep "(MASTER|SLAVE)=" /etc/sysconfig/network-scripts/ifcfg-em?
/etc/sysconfig/network-scripts/ifcfg-em1:MASTER=bond0
/etc/sysconfig/network-scripts/ifcfg-em1:SLAVE=yes
/etc/sysconfig/network-scripts/ifcfg-em2:MASTER=bond0
/etc/sysconfig/network-scripts/ifcfg-em2:SLAVE=yes
$ egrep -v "^$|^#" /etc/NetworkManager/NetworkManager.conf
[main]
[logging]

Why did the mac address of bond0 change ?