Caption in Page adding unwanted 10px to width

Here’s what you can do. There’s a filter at the beginning of the shortcode execution function for the front end that will let you hijack the captions. Returning a non-empty value will stop execution of the shortcode, so if you just process the shortcode the way you want it to be processed and return that result, you can get rid of the pesky 10px of inline padding. Putting something like this in your theme’s functions.php file or in a plugin will work:

function wpse14305_img_caption( $empty_string, $attributes, $content ){
  extract(shortcode_atts(array(
    'id' => '',
    'align' => 'alignnone',
    'width' => '',
    'caption' => ''
  ), $attributes));
  if ( empty($caption) )
    return $content;
  if ( $id ) $id = 'id="' . esc_attr($id) . '" ';
  return '<div ' . $id . 'class="wp-caption ' . esc_attr($align) . '">' . do_shortcode( $content ) . '<p class="wp-caption-text">' . $caption . '</p></div>';
}

add_filter( 'img_caption_shortcode', 'wpse14305_img_caption', 10, 3 );