The one on the left is the UK cover. The book itself is a smaller size and the cover is slightly bluer and darker.
The US book on the left has a tinge more green and is lighter.
The other difference is in the reader/reviewer comments on the back.
UK has quotes from Elizabeth Moon and Karen Miller;
US has Karen again (but a different quote), Publishers Weekly and RT Book Reviews.
I have no idea why there is a difference,
but I suspect there is a reason!