Archive manager and file-roller difference

From firefox 3.0, if I click on a .zip file, it can be opened by Archive Manager (default). Instead, if I click on a .tar, it tries to be opened by the Archive Manager (default) and it appears an error message ...

namefile.rar could not be opened, because the associated helper application does not exist. Change the association in your preferences.

If I change the Archive Manager (default), by /usr/bin/file-roller , then it works perfectly. Why? They are not the same?