A related possibility is that most physical books are professionally typeset with line lengths optimized for comfortable reading. Here's a wikipedia article with examples.

The bilingual storybook is a great example of what happens when the line length is dictated by other factors.

I would also be interested to see if tablet/e-reader users fared better, assuming their device's text/column/spacing settings matched the paper version read by participants.

I have a couple of online ebooks from Amazon and the pages of it get mixed up and skip and are sometimes missing. That would never happen with a physical book. I'm too tired to even deal with it.