Are some couples completely mismatched?

I once knew this girl that worked at Pizza Hut and was definetely dating a loser of a boyfriend. She had just graduated high school and was about to enter college. She tells me of the assault this person has had on her in the past. No offense to people that wear metal on their face and tattoos, but this guy was truly a loser. He never graduated high school and was a druggy, she was not. One night, he hit her for the last time and she finally called the police and he was arrested. She is now a junior in a Kentucky college. Question: how do couples become mismatched from day one? She was so far above his level and I still do not understand how this happened? Do you?