Why cannot I create a file in a folder I own with chmod 655, but 755 works?

I have a backup folder owned by a system user (mongodb) and chmoded 655. When I try to create files in it, it raises a permission issue:

root@maquina:/var/backups/mongodb# su -s /bin/bash mongodb -c "/usr/bin/touch test.txt"/usr/bin/touch: cannot touch `test.txt': Permission denied

Chmoding to 755 enables me to create the file.

I guess I probably need to tweak the privileges (+X ?) but cannot find how.

Can anyone shed some light on this ?

Thank you!