One pixel extra with a width of 100%

I got a textarea of 680 pixels wide. This needs to be made fluid so I use percentages.

The percentages make a perfect 100% width but it seems to render 1 pixel more than it should.

http://jsfiddle.net/XXshn/2/

And what would be wrong with this:

textarea {
border: 1px solid #333333;
background: #ececec;
margin: 0;
outline: 0;
height: 100px;
padding: 20px; /*  */
width: 640px; /* 680px - 2 * 20 */
resize: none; }