effectiveness of power filters.

I have a PS Audio Ultimate Outlet and a couple of Power Premier regenerators.  I got a wild hair and decided to use the distortion meter on the Premier to test what changes the Ultimate Outlet might make in line ahead of the Premier.  Distortion off the wall without the Ultimate Outlet was 4.0 .  Distortion with the Ultimate Outlet in place was 3.8.  Output distortion in both cases was flickering between .5 and .6.   Does this surprise anyone besides me?  I'd have thought the Ultimate Outlet would do more.  Or is it doing just RFI etc.  But isn't that the distortion the whole thing is about?  RFI etc.  Thanks