Reading comprehension is both an art and a science. It is a science because you have to understand grammar and all of the rules that govern the reading, writing and speaking of the language, in our case English. If the writer or speaker does not follow grammar and the rules associated with it then he would be talking or writing nonsense and no one would understand what he is trying to say. Likewise, if the reader or listener has not studied his grammar he will not be able to understand what is being said or written. Communication is a two way process and both players must follow conventions and rules.
Reading comprehension is also an art because the language allows us some freedom in choice of words and the way we construct our sentences. It is here that the writer or speaker can use his creativity in putting across his message in a particular manner. It is up to the reader or listener to make sense of what is being said or written.