Go down to the value section of their pages and click the glossary drop down. Their value calculation for position players is made up of five components (batting, gidp, baserunning, fielding, positional scarcity). Greene has a 15 run batting advantage, but loses 2 in baserunning, 1 in GIDP avoidance, 14 in fielding, and 6 in positional scarcity.