Because Hairston is a righty and the pitcher they threw today was a righty. Henderson has been huge with these matchups and hasn't actually cared at all about who has hit well against who. It is pure righty v. lefty and lefty v. righty.
These matchup deals are extremely overrated, especially in the college level and lower. D1 college players are fresh out of HS where they likely hit 3rd or 4th (in lineup) vs any and ALL pitchers they had to face.
When they start moving up to the pro leagues, specialization can start happening...but damn, don't believe its that big of a deal in college.