At first I create a file and check it's 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 the ACL mask on the file and again check it's 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.

1. What connection does exist between ACL mask and standard group permission?
2. What is the reason for coupling ACL mask and file group permissions? What logic does lay behind it?

The distribution in question is Debian Linux 7.6.