(ROG) world underground is from different save?

first of all there is no category for reign of giants so im putting it here. so i went underground on a new world and when i got there it was already explored and resources taken. After awhile i realized that it was the same underground from another world. I have an idea to how this happened i think its because i deleted the world on the save file then created a new world and it kept the underground info for some reason