I looked this up a couple of years ago for a friend whose son complained of "the words falling off the page", and while I found lots of self-reports, I found nothing remotely credible in the way of explanations.
An optometrist who specializes in visual processing issues identified accommodation and some convergence insufficiency, and suggested he start by trying prism glasses (yeah, I didn't think that was a real thing either, but here's the first explanation I found just now:
http://glassescrafter.com/information/prism-in-eyeglasses.html). Though they helped, child wouldn't wear/ kept losing (very expensive) glasses. Optometrist suggested vision therapy was plan B: a harder fix but a more permanent one.
The VT did fix the "words falling off the page" and the child did start reading a lot more, and a lot less reluctantly. However, he is still not an eager reader: whatever his barriers were, VT provided a notable improvement but not a magic fix. (It did seem to be magic for his drawing, though).
Sorry - wish I could offer better than n of 1 anecdata!
