He's a top power play producer, but he's not in the top 20 at ES over that period.
No, power play points aren't as value, something called "opportunity cost," you can work around the loss of a top power play producer easier than the loss of a top 1C who dominates games at ES.
As good as Giroux was on the PP last year, Flyers were only 15th in the league in PP%.
The season before, when he and Ghost struggled, they were 14th.
2017-18: 15th - Giroux 9-27, Simmonds 11-6, Couts 7-7, Voracek 5-30, Ghost 7-26, Patrick 5-3 [10 SHA]
2016-17: 14th - Giroux 5-26, Schenn 17-11, Simmonds 16-9, Voracek 5-23, Ghost 2-21 [9 SHA]
2015-16: 11th - Giroux 6-21, Simmonds 13-8, Schenn 11-11, Voracek 1-22, Ghost 8-14 [8 SHA]
2014-15: 3rd - Giroux 14-23, Simmonds 14-10, Voracek 11-22, Streit 4-26, Schenn 7-12 [6 SHA]
2013-14: 8th - Giroux 7-30, Simmonds 15-9, Voracek 8-15, Vinnie 8-5, Kimmo 1-19, Streit 4-11 [11 SHA]
2012-13: 3rd - Giroux 6-15, Voracek 8-9, Simmonds 6-10, Kimmo 3-14 [48 games] [3 SHA]
2011-12: 6th - Giroux 6-32, Hartnell 16-7, Simmonds 11-16, Jagr 8-12, Kimmo 4-17 [9 SHA]
2010-11: 19th - Giroux 8-11, Richards 5-16, Carter 8-9, Briere 6-9, Kimmo 1-15, Pronger 3-11 [5 SHA]
What seems to drive success on the PP has been less how well Giroux has played, than how well and how deep the goal scorers were on the PP - in a sense, it's more of a group effort than 5x5 play.