If "the" is specific and "a" is non-specific, why do people say "This is an easy question," if they're referring to a specific question? Wouldn't it make more sense to say "This is the easy question"? No, because it's not the only easy question in the world. It's only one of countless easy questions that could be asked.
If you know that there is only one easy question that we are talking about, for example on an exam, that is a specific easy question and you would say 'This is the easy question.' (the only easy question on the exam)
But if someone just asks you a question and it is an easy question, you would say 'This is an easy question.' It's not the only easy questionsomeone could ask you, so it is an easy question. (one of many easy questions a person could ask you)