Your variable says to get the trigger's resulting filename for the active job and the trigger with the given ID, so I think what you're getting is the file that fired that trigger the most recently. The other alternative provided is to use the last trigger to trigger, which I'm still not sure would get you the file that actually triggered it as opposed to the most recent one, and might not even be guaranteed to be the right trigger if you had more than one...I'm not sure how to do what you're looking for, because the last time I needed to do something similar I got to about the point you have and then decided I would just make sure it triggered (at least) once, but I would handle the actual files the same way I would if I were running it on a timer and processing everything that was present instead.
Sorry I can't help more than that.