I saw this thread the other day and I'll be honest, I don't think your resolutions are matching up. Within the default UI the
loot roll windows should have a set X,Y coordinate in which they never move from and those coordinates are not randomly
chosen when you create a new character. Obviously, if you're using some sort of add-on then this could change the position.

I've been wrong in the past but, if you could double check this for me... turn on repeating with SHIFT+ALT+R and compare the
mouse cursor's position on your main screen to your slave screen from while moving the mouse from the upper left hand
corner to the bottom right hand corner of the screen.