My guess is the box was updated independently of the backs, so, yes, sort of random. By extension, I don't think the ISBN will have any bearing on the back design. Then you have to consider the old stock is still going to turn up in shops and online listings, depending on how slowly they turn their merchandise.
I bought my copy in a head shop in Las Vegas manned by uninitiated teenage staff, so it was luck of the draw for me that I got the newer multi-colored backs. I don't like the green/white backs, but that is because I don't like that color scheme on anything.
I wonder what prompts LoS's whimsical back design changes? They did the same with their printings of the Visconti Tarots--which I finally resigned to accept the newer back design. Publishers should understand that back designs are just as much part of the deck as the faces . . . very consternating when you are not sure what you are getting when you purchase something.