OK, well if you were on PM 5 then I would wonder if my sorting algorithm weren't working correctly. But since you're on PM 4.6.x and it does not have any adjustments for sorting, I can say that all folders/sub-folders are sorted the same, so I'm guessing that something has happened on the NAS itself to those sub-folder names.
Are they just completely random? Do they sort any better in an instance of Windows Explorer's folder tree?
-Kirk