xfce4-settings-manager by default is placed in the center of the monitor, but if you have two monitors and the default one is e.g. on the right, then settings manager will be placed on the left one.
*** Bug 13484 has been marked as a duplicate of this bug. ***
What do you mean by "default"? I think that this behavior of placing things on the left-most display is fairly common - it also used to be the case in the panel. Not sure which apps would handle that differently.