Hi there, apologies for the very late response!
The simple answer here is that we tend to use the [a] where we know that something is different about the image but where we don't necessarily have any further information. If something is over- or under-dumped, bad, hacked, modified, translated etc. then it would get one of the other tags, if it's a known different release that would hopefully be reflected in e.g. date or publisher fields.
We're always trying to improve and rely less on the generic [a] tags, but it's obviously a huge project. If you have an interest in digging further into the details of any particular system and posting your findings, that would be great!
