Format may have a lot to do with it:
- Some people thrive on reading printed paper or hardbound books in a linear fashion.
- Others excel at retaining information gathered by hopping around quickly through a digital text, as well as the additional options to easily read for a short time (in line at the bank, for example) while also marking one's place as provided by most e-readers.
- Still others find "reading" audiobooks is the best method to ingest and retain knowledge long term for both fiction and non-fiction works.
- Graphic novels blur the line a bit – in some cases retaining key description and dialogue, and replacing a bulk of the text with pictures, which can convey meaning equally, or in some cases, with much greater understanding.
- While there are precious few titles available, even "micro-chunking" a book, by having it sent over time as a series of emails is now an option for some books – see http://www.dailylit.com/ for one example – which again provides an ease of access/lack of routine change element that can bring regular reading to those who otherwise can't push through a 200 page paperback.
Overall, I think we're finding that as scientists discover more details about how different personality/behavioral types learn, the options to accomodate these different types is expanding into areas that aren't thought of as traditional "reading". And as this fragmentation is then perceived (perhaps wrongly) by some as an inability to "focus on reading" in a specific medium.
Thus, it may be the case that people are not losing focus while reading in a given format, simply that their ability to focus while reading is tied to an alternate reading method.