The beauty of Pew's research is that they make their methods known too.
The wording of the question used was: Please tell me, how often do you read scripture outside of religious services?
I think that to many a JW this would include the daily text, any scripture the regurgitate in FS, any scripture they read while preparing for the meetings. Don't forget JW are expected to do all of that....and 12% apparently doesn't even get that far.
Also keep in mind that JW are made to feel more compulsion and guilt when it comes to religious behavior. It's one thing to privately not read the Bible every week despite the GB saying you must.....but actually admitting that to yourself by saying it out loud, and giving a bad witness to this worldly surveyor... that's even harder....