chmod behaviour

[RuceLee]

Hello, fellow subscribers

Recently I was playing with directory permissions on a ext4 filesystem and I have stumbled
at a situation when setting up setting up directory SGID behaves not as expected, at least for me.

For example:

chmod g+s my_directory works the same as chmod 2755 my_directory

On the other hand:

chmod g-s my_directory doesn't work the same as chmod 0755 my_directory

the latest doing absolutely nothing  to remove the intended SGID bit.

The latest package I have is coreutils-8.4-13.el6.x86_64

Thank you for your thoughts.