weird jquery problem in IE8/7

Question

hello.

the problem starts when i use

#mydiv{
    margin:0 auto;
}

all other browsers just works fine. this is an example

alert($("#mydiv").position().left); //output 0
$("#mydiv").animate({'left': '-=500'});
alert($("#mydiv").position().left); //output -499

how can i solve that? is it a common ie problem? is there any solution for this?