mjred,
I am not sure what causes BoDog to give someone their better favorite line vs. their better underdog line. I lose about 1/2 point on every underdog bet there compared to a friend of mine who I referred there. We both tend to bet underdogs and unders more than favorites and overs. There are a few differences in our playing styles. He bets very, very small while I bet 100-200 per game. He deposits a relatively large amount compared to his wager size to get a good bonus while I deposit one wager amount at a time. He bets most of his action at BoDog while I only play there from time to time. I'm not sure why those differences would cause us to get different lines, but for whatever reason, he gets the softer underdog lines.
