When scanning existing images, we should scan in chronological order, starting with the oldest images.
If we scan in alphabetical order, and we need to split up the scan into multiple runs to fit into our 10M/month limit, new files will be uploaded in the previous runs. So, if we start with files starting with 'A', any new files starting with 'A' won't be part of that scan.
Scanning in chronological order helps with that. New uploads will get scanned last.
I believe there are some edge cases: for example, older revisions of files that get undeleted. I'm not sure if that's even possible or how frequent it is, but my guess is that it's much less frequent than new uploads.