Comment 5 for bug 889394

Well as you correctly pointed out, XScreenSaver uses its own DPMS settings and ignores the settings of the Xserver. The power manager instead uses the Xserver's DPMS interface, so the two can never really correspond. So this really isn't a bug in xfce4-power-manager.

However, we might in a future version of xfpm include a tab to handle screensavers/lockscreens, where the user can select which software he wants to handle the locking. When XScreenSaver would be selected there, we could make all the DPMS and blanking related controls insensitive so that the settings can only be edited in XScreenSaver's own settings UI.

But that's about everything that can be done about this bug imo. Making xfpm override XScreenSaver's settings isn't really possible afaik.