update_post_meta updates to previous revision value

I have a couple of values (in meta fields) set when creating or updating a post. In my functions.php I iterate through all the these values when saving and update the value. The problem is that when changing to a new value, the code below will set the value from the previous revision.

$captionData = $_POST['caption_value'];
if(get_post_meta($post_id, 'caption_value') == "")
  add_post_meta($post_id, 'caption_value', $captionData, true);
elseif($captionData != get_post_meta($post_id, 'caption_value', true))
  update_post_meta($post_id, 'caption_value', $captionData);

Example:
Revision 1: set ‘caption_value’ to “foo” -> post_meta remains blank
Revision 2: set ‘caption_value’ to “bar” -> post_meta returns “foo”
Revision 3: set ‘caption_value’ to “foo2” – post_meta returns “bar”
etc…

Elsewhere in the theme the code below works like a charm:

$data = $_POST[$meta_box['name'].'_value'];
if(get_post_meta($post_id, $meta_box['name'].'_value') == "")
  add_post_meta($post_id, $meta_box['name'].'_value', $data, true);
elseif($data != get_post_meta($post_id, $pre.'_value', true))
  update_post_meta($post_id, $meta_box['name'].'_value', $data);
elseif($data == "")
   delete_post_meta($post_id, $meta_box['name'].'_value', get_post_meta($post_id, $meta_box['name'].'_value', true));

I’m not sure if my problem is in understanding update_post_data, the $_POST variable or even if it has something to do with $pre (a variable which never seems to be set anywhere.

Any guidance will be greatly appreciated!