Another example of the same principle would be casting pillar of light against someone with the tough -2 trait. Suddenly it has become impossible to stun them. Either you apply the trait to the dice roll, making it impossible to roll higher than a 10 (which is less than the 11 needed). Or you apply it to the target number, making it require that you roll at least a 13 on a twelve sided die, which is also impossible.
Both result in the same thing. Modifiers can make the possible impossible, such as in the case of pillar of light's stun, as well as being able to make the impossible possible, such as with spike pit trap's stuck. This is perfectly acceptable within the rules as written.