It seems to have been automatically done in a completely random and nonsensical order so it appears: Volume 4, Volume 2, Volume 1, Volume 5, Volume 3, Volume 6.
I can’t see the option to do this when on manage inventory > edit (for the parent listing) and I raised a case with seller central who were no help.
It is set up as a text variation, as the titles aren’t quite as simple as I wrote above (the variations are actually ‘Original Game’, ‘Volume 2’, ‘Xmas Edition’ etc; but they are in a random order)
It’s usually the order in which they appear on the variation page. I can’t remember if there is an option to move them. The other option is to delete and redo them in the correct order.