What is Arrieta's away from Wrigley Field?
This is a fair comparison. You are pitching all around Major League baseball seeing many different venues.
Let's look at Clayton Kershaw's career numbers graphed out over ERA and Home/road split:
http://www.fangraphs.com/splits.aspx?playerid=2036&position=P&page=0&split=ha&type=full
You notice the huge differential in most seasons between home and away. For instance, Kershaw's ERA at home is around 1.5 and his ERA on the road is around is 3 this year. Most seasons saw his home ERA be anywhere from 1/2 to full run lower at Dodger Stadium. Greinke has similar numbers since coming to the Dodgers.