My template page can't get the right width

Hi everybody,

Now that I understand more the way to customized the pinboard theme, I’m trying to create a template for my home page. I followed this post : http://codex.wordpress.org/Stepping_Into_Templates.

It’s working fine…but I can’t get my new index page (using my template) with the same width that the others. I use the default layout template without sidebars (the penultimate choice in Theme option > Layout), normaly the size of my general content frame must be like 760 px…but it is 1140 px.
Any ideas about this issue ?

Thanks for your comments,
Eric