Try looking at the image and margins at:
Windows & Caption Buttons > Aero > DWM Window > Frames > Aero Peek Window > Normal > Right
margin parameters are (Left,Top,Right,Bottom)
There's also Windows & Caption Buttons > Aero > DWM Window > Frames > Details > Reflections > Aero Peek Background
Don't see anything wrong there, and there's nothing in "Aero Peek Background". No image there, yet the reflection still exists.