Ah, I see what you mean. I'll try to do it that way, but it does seem a little precarious because the moment you sort by something else (even by accident) then the numbers reassign and you've lost the original order you brought into the list. There doesn't seem to be (as far as I can tell), a way to really make that order fixed (at least not the same way you can with images that have been merged into a single document)
And it makes perfect sense really that the order added, even if done on a more granular level, isn't the way to go b/c that's just now how the OS is thinking about things.
This kind of sorting can be more useful than you might realize. As an example, when merging items, sometimes it isn't easy determine whether two sequential pages should go together (this is esp the case in governmental records where a memo may have several attachments. Perhaps you cluster 8 images together, and it is only when going back and transcribing the memo you realize that what looked like a separate memo is actually, rather, an addendum w/in the memo- and it is important for you to be able to preserve these together. But by now you've lost that original order so you have the joy of checking each item from that archive file and figure out which item this image should have been attached to....
On a related note, the image previews are always centered- which means that usually the headings are cut off, which in turn makes it harder to determine whether to merge items together- especially when page after page of block text and only the top three lines indicate whether it is a new item or continuation of the last one. Is there a way to adjust that? Otherwise I have to keep navigating into an item and back out again