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What relationships do tie ACL mask and standard group permission on a file?

At first I create file and check its standard permissions and ACL entries:

$ touch file; ls -l file; getfacl file
-rw-r--r-- 1 user user 0 Jul 30 16:26 file
# file: file
# owner: user
# group: user
user::rw-
group::r--
other::r--

Then I set ACL mask on the file and again check its standard permissions and ACL entries:

$ setfacl -m mask:rwx file
$ ls -l file; getfacl file
-rw-rwxr--+ 1 user user 0 Jul 30 16:26 file
# file: file
# owner: user
# group: user
user::rw-
group::r--
mask::rwx
other::r--

Note that along with ACL mask standard group permission on the file also changed. Why does this happen? What do developers meant to show by this change? What connection does exist between ACL mask and standard group permission?

The distribution in question is Debian Linux 7.6.