I'm sure you are not a mathematician, so you can be forgiven for thinking the order is random. It certainly is not. It may not be the order you wish for your particular use--that is your call.
But most search engines return their results in order of relevance, which is what ours does for a "normal" search. So in the case you describe, if you had an image tagged "beauty" and "BMW" it would appear above one just tagged with one of those terms. If you include other fields in the search besides tags, the keywords appearing in multiple of those fields will add to the relevance of the results.
Your scenerio presumes some knowledge of the search subject, which you have, but Zenphoto could not possibly presume. That is why you really need to apply a custom sorting to get what you want.
Otherwise, who is to say that the "name" or "description" or maybe the tags are not the order you would want the items in.