Where do the extra pixels come from?

Here is a site I completed recently:
http://www.janetphillips-weaving.co.uk
The main nav menu has four li in an ul displayed as blocks with
background images and the width set at 100px. I didn't put any spacing
between the items but when I measure them the four take up about 410px
width. I can't find anything in any css to account for the extra px.
where have they come from?

Puzzled,

Tim w