canvas mouse event position different than cursor

I am starting to experiment with canvas, this might be a basic issue but can not find the problem here.

My mouse cursor is slower than the line I draw on canvas with the mouse event coordinates. What am I missing here?

Fiddle

P.S.- Same problem if using jQuery's mouse event: http://jsfiddle.net/2q4Sd/2/

window.addEvent('mousemove', function (event) {
    draw(event.event.clientX);
});

function draw(mouseX) {
    ctx.beginPath();
    ctx.strokeStyle = "black";
    ctx.lineWidth = 1;
    ctx.moveTo(mouseX, 0);
    ctx.lineTo(mouseX, canvas.height);
    ctx.stroke();
    ctx.closePath();
}