That means there are, actually, just three different maps per seed number: "Mixed" (some positive, some negative), "Neutral" (never influenced), "Influenced" (in each case positive OR negative). However, it's easy to prove that positive/negative influences ultimately lead to the "same" map, with the (positively/negatively) influenced islands switched against their counterpart. In game setup, there are four different influences to choose from: neutral, positive, negative and mixed. If you allow for positive and/or negative influences in the map setting, it is possible that you encounter islands twice: The Twin of the neutral island then features (for example) an old warhead or some old facility.
Hoover over each setting for a detailed description. In the Game settings tab you are able to customize theĮnvironment, as well as start-, and win conditions.