各位大侠，问个问题


 var moveX,moveY,l,t;
           var move=false;
           $("#head").mousedown(function(e){
                move=true;
                e=window.event?window.event:e;
                l = parseInt($("#Message").css("left"));
                t = parseInt($("#Message").css("top"));
                moveX = e.clientX-l;
                moveY = e.clientY-t;
                $(document).mousemove(function(e){
                    if(move){
                        e=window.event?window.event:e;
                        var x = e.clientX-moveX+"px";
                        var y = e.clientY-moveY+"px";
                        $("#Message").css({left:x,top:y});
                    }
                })
                $(document).mouseup(function(){
                    move=false;
                })
           })
刚点击的时候会偏移，第二次在移动就不会了，什么原因呢？是css的原因吗