After the initial standard setting, forms are pre-equated using Item Response Theory (IRT). To further ensure consistency among exams, IRT equating places questions with different questions on same scale. That scale is based on the Item Characteristic Curve (ICC) -- the relationship between the difficulty of a question and the ability of an exam taker. The probability of correctly answering a given question depends on an exam taker's ability and the characteristic of the question.
Because all exam forms are pre-equated, the number of questions answered correctly to earn a passing score varies depending on the exam form and when the exam was taken. That would not be the case if we set an arbitrary passing score (such as 70 percent) for each exam. So even though one exam form might vary slightly in difficulty from another, these quality procedures ensure a passing standard.