Question: Why is item difficulty for a hard question 0 (or 0%) and item difficulty for an easy question 1 (or 100%)? That doesn’t make sense.
Answer: You’re right. It’s the opposite of the way we normally think about scales. It works out this way because item difficulty is a measure of the percent of test takers who got the question right (0% for a hard question and 100% for an easy question). So it should really be called item easiness. Note: Some people deal with this problem by defining item difficulty to be “1- percent-correct.” Then the numbers look right. But this can be confusing unless you make it clear that you are using a different scale.