FIP is a highly flawed statistic, and Mark Buehrle is exactly the pitcher it tends to under value. Because it doesn't take into consideration any batted ball besides HR, it undervalues pitchers who have low strikeout rates (like Buehrle) but overvalues pitchers who are batting practice to the other team (like Lackey, whose high LD% is a trend, not an outlier, making his high BABIP deserved). While it does add a league-specific value, it still has definite shortcomings.
WAR and FIP are both useful tools, but they're not good measures of a player's value by themselves.