Well, surface area has a lot to do with the terrain. If Mars were a perfect sphere (which it's not), then it wouldn't take much water at all to have a high surface area water::land ratio
Yes, I am not sure if they calculated this based on Mars being a perfect sphere or not. I did find this interesting image of all the water on Earth, compared to the size of Earth: http://water.usgs.gov/edu/gallery/global-water-volume.html
What a incredible image! If a picture is worth a thousand words, this one is a very representative one. Very well done. Congratulations to the team.