1) Every time you hit a sacrifice fly, you're not charged with an at-bat, but you are charged with a plate appearance. So your OBP will drop for a sac fly, even though your batting average doesn't change. If a player hits more sac flys than he recieves walks, then his AVG will be higher than his OPB. This happened to Alfonso Soriano at the beginning of last year.
2) The two averages are not based on equal samples. In other words, since batters generally face more righties than lefties, the batter's average is going to be a little skewed towards righties. For example, if a player goes 3 for 27 against righties (.111), and 4 for 10 against lefties (.400), his combined average isn't going to be .256, the average of his two averages. His extra at-bats against righties are going to drag him down to 7 for 37, or .189.
Hopw rhia hwlpa.
Or with my fingers on the right keys, hope this helps....
[ March 16, 2002: Message edited by: IngredientX ]</p>