ModalPopupExtender Center Alignment

I have an extremely basic ASP.NET page that relatively positions an HTML table in the middle of the page. The table contains an ASP GridView control and a link button. The link button is the target control for a ModalPopupExtender. When I click the LinkButton, the Modal Panel is displayed, however, it dispalyed center position in my screen.

But this page is using in master page. now this is not display in center screen. dispaly in child window center position.

I want the modal panel to be centered in the middle of my screen. What could cause this behavior?