Handle compress command error messages on AIX

I am working on production issue and unable to reproduce error message for compress command.

Current Solution

We have case-esac block in shell wrapper that handles messages as follows;

case "$MSG" in
    "This file is not changed; compression does not save space.")
            echo "$MSG"
            ;;
    *unchanged*)
            echo "$MSG"
            ;;
    *)
            echo ERROR
            ;;
esac

Problem Scenario

The compress command when called through wrapper (for some files) produces error message (let say, Type 1);

 -- file unchanged

However, executing the same command (for same set of files) on command line produces different error message (let say, Type 2);

This file is not changed; compression does not save space.

Can someone help me understand this behavior? If possible, suggest reproducing Type 1 error message