CentOS 7.2 VM ACL mask problem...

CentOS 7.2 VM ACL mask problem... why mask is Not same as group permisions? my book says mask is min permissions set my normal group permissions... well? why shell Do Not show effective permissions on getfacl...?

group::r--

...

mask::rw-

==============================

[tux@server1 ~]$ touch acl1.txt

[tux@server1 ~]$ ls -l acl1.txt

-rw-rw-r--. 1 tux tux 0 Aug 23 18:23 acl1.txt

[tux@server1 ~]$ chmod 644 acl1.txt

[tux@server1 ~]$ ls -l acl1.txt

-rw-r--r--. 1 tux tux 0 Aug 23 18:23 acl1.txt

[tux@server1 ~]$ setfacl -m g:sales:rw acl1.txt

[tux@server1 ~]$ setfacl -m u:bob:rw acl1.txt

[tux@server1 ~]$ getfacl acl1.txt

# file: acl1.txt

# owner: tux

# group: tux

user::rw-

user:bob:rw-

group::r--

group:sales:rw-

mask::rw-

other::r--

[tux@server1 ~]$