The documentation of jQuery.animate states that

The only required parameter is a map of CSS properties. This map is similar to the one that can be sent to the .css() method, except that the range of properties is more restrictive.

So, why does this work

$('#con div').css( {
        top : function(i) {
            console.log(i);
            return (i * 500) + 'px';
        }
    }
);

and this doesn't?

$('#con div').animate( {
        top : function(i) {
            console.log(i);
            return (i * 500) + 'px';
        }
    }
);

The console shows that the function is being evaluated for css, but not for animate. Am I missing something?

By the way, I'm using 1.4.2.