Had a look in the download folder and surprisingly both folders had the same number of files, but slightly different disk size used. I tracked it down to one jpg. AeroScenery seems to have downloaded something, but when I try to look at it, it is not a valid jpg. I assume it created the name but then had nothing to put in it
FSET used a queue system for any images it wasn't able to get straight away, although it used to end up with quite a few in the queue not just one. So I don't think it did any retries on the first run. Once it finishes all the other images, it returned to the images in the queue and did a second attempt. I never had an instance where it failed to get all images, so it seems to just be a timing issue.