You must have richer soil where you planted that one. There is no set genetic limit on how big these things will get. If there were it would be called "determinate", and there are tomatoes and squashes like that (well, only c. Pepo squashes AFAIK) but no watermelons as far as I'm aware.
RE: One Watermelon Plant Took Over Half my Garden (It's only supposed to be 6 to 8 feet)