I think it could be because:

When you run it from the shell (manually) your environment is all setup. So you can use simple commands (ie tar, gzip, ls) as opposed to typing those commands with the full path (ie /bin/tar, /bin/ls).

However when you run it from a cron, you are not logged in so there is no environment setup. Therefore when the machine reads the command tar or ls, it has no idea what your talking about. Try using the full path in your cron script