Some of it? Try a lot of it. Again, he was in an absolutely ideal situation. Not just in terms of zone starts, but match ups. He wasn't just in a 3rd pairing role. He was in a sheltered 3rd pairing role, seeing, by far, the easiest match ups. You keep talking about shots generated, but you seem to hand wave that. Also, Anaheim only lost 20 games in regulation period, so overall I'd say that stat is fairly meaningless. He played a little over half of the season, and he played most of the games earlier on when Anaheim was on a roll. A little context, please.
The entire problem is that he was so sheltered, and that can't continue. When Anaheim would have been trying to weather the storm, Sami is not the guy who would have been on the ice. When the team was facing the opposition's top players, Sami is not the guy who would have been on the ice. I'm not saying the shot differential with him on the ice isn't important, because it is, but you make it out to sound like he drives the play more than anyone else the other way, and that just isn't true. Imagine if Lindholm or Fowler were on the ice in those situations, and as it is they both drive the play in much tougher situations.
My biggest issue with Sami really isn't about the good or the bad, it's about the role. Boudreau sheltered the crap out of him, and for a coach who likes to roll the lines I find that to be a concern. BB hasn't historically been a huge match up coach, certainly not at the level we saw from RC, but he worked very hard at getting proper match ups for Sami. To me, that says the coach has some concerns about Sami's ability to handle more at this point. I think next season, we'll see him get a little more away from that, because he'll want Sami to carry a bit heavier a load.
Edit: and none of this is about Sami's skill. He is undeniably an extremely talented young player, with, I think, great offensive upside. Smaller defensemen like him typically take a bit more time before they are ready for a full time job(I actually think he's ahead of the curve at the moment). I think Anaheim is doing a good job bringing him along the way they are, and I actually think the results point to that too. I think he'll soon reach a point where he can get a more standard 3rd pairing role, and we'll see him hit another level in terms of offensive contributions and PP performance. I think he has the potential for a big breakout season, either this next one, or the one after(I think he'll be a full-time player next season, either way).