dialog position center isn't center

I'm noticing that the dialog window is opening where the "top" of the
dialog window is at the center of the page rather than the whole
dialog window. These are the settings we're using:
$("#modal").dialog({autoOpen: false, autoResize: true, title:
"Employee Questionnaire Status", width: 450, minHeight: 400, modal:
true, overlay: { opacity: 0.5, background:"black" }});