This can and does happen, for no real good reason. Twice in one month, I had the Captain line up with the wrong runway. These seperate incidents happened at airports with parallel runways, and both happened because the flying pilot lined up with the runway they were expecting, not the runway we were assigned. At around 5 miles out on a visual approach, the non-flying pilot often has his eyes inside, either running the landing checklist or switching to tower frequency, or just verifying the picture inside. On both occasions I noticed the error around 5 out, at the point when we were rolling out onto final from base or modified base leg. I called the error, the captain corrected, and CRM saved our certificates once again. And likely something very similar happened in the situation given. Only reason I can think that such an event would be such a big deal is if they lined up on final for a runway that was also being used for landings, in which case there would have been a potential or real loss of seperation with anoth arriving aircraft.

In short, it happens. We all miss at some point. This is why there are two sets of eyes up front.