Answer: The chart is lacking any real data beyond a few pretty bars, and that makes it pretty useless IMO.
And that's why I went and got the data from the source.
The chart was generated by Readmill, the social reading platform, so it's not a completely accurate picture of the mobile reading experience. The data is drawn from a userbase in the tens of thousands, all of which were reading on either an iPad or iPhone. Naturally that means that this chart says nothing about the Android reader (or any other platform for that matter).
And yes, anyone who describes this as smartphone vs tablet is simply wrong.
But the data does show that Readmill's iPhone users do average more time spent reading than an iPad owner. For example, they're 45% more likely to open the app, and they average 40% more time spent in the app. Readmill's iPhone users also tended to spend 10% more time per book and read 15% more ebooks. They leave 40% more comments, and copy 20% more quotes than iPad users.
iPad users started 18% fewer books and they finished 20% fewer books than iPhone users. According to Readmill, their iPad users are also inclined to spend twice as much time on the web as their iPhone users. That should really come as no surprise given that tablets tend to depend on Wifi for web access while smartphones tend to depend on the much more expensive mobile data plans.
And last but not least, Readmill's iPhone users are also more social. They follow 30% more people and average 65% more followers.
Interesting data, no?