Even if they understand my writing, they usually do not want to read about peptic ulcers. We know that lots of areas factor into readability beyond limiting big words and making sentences shorter.
Motivation is one big factor.
A person who is a great reader, but who has no frame of reference on say, working a VCR may not be able to figure out how the darn contraption works. A person who has limited reading skills, but has a child diagnosed with leukemia may pour over information, learning complicated terms and weeding through lots of 3-syllable words. He loves to play the horses. He is motivated and worked out a system of using yellow highlighter to mark his place on the race form. Yet, this same guy had a terrible time chosing a new health care system at work.