How is it that people can have problems with comprehension when there only language is English?

When something is worded in plain English, punctuated and logically written it should be understandable, yet I've seen numberous examples of people not being able to understand questions. It's not the way the question is put, since most people will answer the questions but a few seem unable to understand what is asked or even be able to tell that it is a question, which is very, very weird. Is this some form of illiteracy?