Ah – I see what you mean about the distortion.
WordPress natively adds 10px to the caption width in order to give the image a border look – which in my opinion is an old, over-used style.
You can remove this extra padding by adding this PHP snippet using a plugin like this: https://wordpress.org/plugins/code-snippets/
function generate_remove_caption_padding( $width ) {
return $width - 10;
}
add_filter( 'img_caption_shortcode_width', 'generate_remove_caption_padding' );
I would add it to the core of GP, but WP.org most likely would reject it as it alters default WordPress behavior.