For many of us, reading a hard copy version is more comfortable than reading something on a computer screen for an extended period of time. Most of us are used to reading stuff or taking tests in print. In short, what you are experiencing is natural.
It is good, though, that you noticed this issue and made note of it. You have found a weakness and can now work on it to improve! For starters, definitely do all practice test session using a standard chair and table (beds, couches, and fluffy chairs should not be involved). Another method that might help is to prop the OG books upright in order to mimic the way the questions are presented on a computer screen. For me, another good adjustment method was practicing with Jeff Sackmann's math sets (PDF format) and
GMATClub tests (online).
You want to be completely comfortable with taking the GMAT under actual test conditions, so do everything possible to adjust in order to make this distraction go away.
Hope this helps!