$(s).position({ my: “center+0 top”}) is being positioned with offset

$(s).position({ my: “center+0 top”}) is being positioned with offset

I am using this plugin http://api.jqueryui.com/position/ to position element. If I do
$(s).position({
my: "center top",
at: "center top",
of: '#main_container',
});

$(s) is being positioned by the center

but if I do
$(s).position({
my: "center+0 top",
at: "center top",
of: '#main_container',
});

it moves a bit right from the center.