Default Gallery Output

I was trying to work on a theme. I found out that when generating a default gallery out put, the gallery shortcode adds some styling information just before the images like this

<style type='text/css'>
			#gallery-1 {
				margin: auto;
			}
			#gallery-1 .gallery-item {
				float: left;
				margin-top: 10px;
				text-align: center;
				width: 33%;
			}
			#gallery-1 img {
				border: 2px solid #cfcfcf;
			}
			#gallery-1 .gallery-caption {
				margin-left: 0;
			}
		</style>
		<!-- see gallery_shortcode() in wp-includes/media.php -->

I looked into the gallery_shortcode in wp_includes/media.php and found this code:

if ( apply_filters( 'use_default_gallery_style', true ) )
		$gallery_style = "
		<style type='text/css'>
			#{$selector} {
				margin: auto;
			}
			#{$selector} .gallery-item {
				float: {$float};
				margin-top: 10px;
				text-align: center;
				width: {$itemwidth}%;
			}
			#{$selector} img {
				border: 2px solid #cfcfcf;
			}
			#{$selector} .gallery-caption {
				margin-left: 0;
			}
		</style>
		<!-- see gallery_shortcode() in wp-includes/media.php -->";

I feel that instead of adding border to the #selector the borders should be added to .gallery-icon. By choosing to add it to #selector which will be gallery-1 or gallery-2, we make it difficult for theme authors to change the style of gallery output.

I was wondering if someone can explain to me why WordPress is using #selector to add borders to images in gallery and not a class like gallery-icon or gallery-item.