As we learn and witness repeatedly throughout the film, Abby doesn't just struggle with the truth — she's a pathological liar. Prior to her homecoming, she spent several years in Arizona conning a local social worker. The "cycle," as her sister refers to it, cost Abby's mother (literally) dearly, forcing her to have to sell the motel. The audience learns this fairly late in the story, and Abby's seeming uncovering of the truth feels like redemption.
In the final few seconds of the film, after Abby appears initially distraught by the revelation that she's just sent an innocent man to prison, we see her watch her reflection in the mirror (a tangible form of the truth twice-removed). It's a subtle, blink-and-you'll-miss-it moment, but her frown begins to curl into a smile as she — and the viewers — realize she's relieved, even happy, to be a part of a lie. The scene is punctuated by an old song crooning, suddenly and from somewhere unseen, "sometimes, I get so lonely, sometimes, I feel so blue..."
Abby (a journalism student, it's worth noting) never really wanted to find the truth. She wanted to sift through a game of potential truths and illusions in order to occupy the void that compels her to lie on a pathological level. That's exactly what she does, and what the viewer does with her, throughout the film. Its one of the many ways in which Albert Shin makes his commentary on today's "true" crime-obsessed society known, and that commentary is delivered as much by what he doesn't show as what he does.
