When a player tries to teleport to another player in a different world (Nether/Overworld) it teleports him to the second player's coordonates, but in the first player's world.

Example : 
Player1 : x = 0, y = 0; World

Player2 : x = 100, y = 100; World_Nether

Player1 does "/tp Player2"

Player1 : x = 100, y = 100; World

(Player2 unchanged)