Why Javascript centering does not work?

Why Javascript centering does not work?

When I select the option to focused with Javascript, I put the "pop up" on the bottom of the screen. Attached screenshot to better illustrate what is stated here.

In this example i´m using
Pop Over Size: 650px (W) / 400px (H)
Pop Over Position: Left: 30% / Top: 20%
Pop Over Margins: All in 0px
Without Javascript to resize and center the popover

http://www.jeansteamstores.com/wattwash/

But i want to center very well, noy only in my big pc screen.

What can i do?