Post custom attribute vs Page custom attribute

I’m a bit confused now.

I thought get_post_meta was used to retrieve page attributes, while get_post_custom_values was used to get post attributes.

Is custom attributes retrieved diferently from a post than from a page?

The following code does not output anything:
echo get_post_meta($post->ID, 'show_categories', true) . '<br />';

But this code does out put the attribute value:

$categories = explode(",", get_post_meta($post->ID, 'category_id', true));
echo $categories[0];

I just don’t get it.