When using a command to testfor an entity in a specific coordinate using the x y z sections in a selector, instead of checking only in that one coordinate, all coordinates are checked, resulting in always finding the entity, even if they are not in the specified spot.

Test:

1.Take note of your coordinates, and use them here: /testfor @e[type=Player,x=X,y=Y,z=Z].
2.Move away from the coordinate and test again, and it still finds the player.
3.This also occurs with all entities.