The fare class priority on LH would explain why I have been fairly successful on op-ups. I am often on higher fare classes since I'm not staying weekends and often fly out Sunday and back on Friday. My last op-up I was on a U fare that I bid on a PE upgrade and got it, then when I boarded, they op-upped me to C. Combined with the extra miles for the PE fare, it was a pretty good use of $175.