As a general rule, there are less left handed pitchers. So lefties face lefties less often and have less at-bats to adjust to them. There are probably other reasons also.
As of the 2014 season, the Atlanta Braves have 16 right handed pitchers.
I'm not sure if this is what you are talking about but... Right-handed pitchers do better against right-handed batters than they do against left-handed batters. And left-handed pitchers do better against left-handed batters than they do against right-handed batters. Conversely, right-handed batters do better against left-handed pitchers than right-handed pitchers and left-handed batters do better against right-handed pitchers than left-handed pitchers. My dad explained to me once why it worked out this way, but I don't know if I fully understand it. Apparently, the horizontal motion on a pitched ball is such that, for a right-handed pitcher, the ball moves to his left, or to the batter's right. If you are a right-handed batter, the ball is moving AWAY from you, but if you are a left-handed batter, the ball is moving TOWARD you. And, apparently, it's easier to hit a ball moving toward you than to hit one moving away from you. Personally, it's incredible to me that ANYONE can hit a pitched ball, even when it's only going 70 MPH, regardless of which way it's moving, so maybe that's why the explanation didn't make sense to me.
As of the 2014 season, the Colorado Rockies have 9 left handed pitchers.
Baseball pitchers are only referred to as a Southpaw if they're left handed, it's more common for left handed pitchers to be referred to as a Southpaw but it's just a general term for anyone who is left handed.
As of the 2014 season, the Texas Rangers have 5 left handed pitchers.
As of the 2014 season, the Baltimore Orioles have 5 left handed pitchers.
As of the 2014 season, the Cincinnati Reds have 5 left handed pitchers.
As of the 2014 season, the Cleveland Indians have 5 left handed pitchers.
As of the 2014 season, the Miami Marlins have 7 left handed pitchers.
As of the 2014 season, the Milwaukee Brewers have 5 left handed pitchers.
As of the 2014 season, the Oakland Athletics have 8 left handed pitchers.