Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
My Teemo build is based on an even split between Damage and Ap with a large amount of At Speed. The point on being able to tailor your items on if the opposite team is building Magic Resist or Armor.
The Last 2 items are really dependent upon if the other team is building up Magic Resist or Armor.