I have taught the Kaplan course for 7 years and find that the practice tests are very reliable. Individual's scores will vary depending on how they deal with test anxiety, but overall the CATs are excellent predictors. Remember, on the real GMAT it takes 40 points to establish a statistically significant difference between two scores. That means that if you get a 650, your actual score is between 610 and 690. Personally, I have taken many practice tests and always score within a 30 point range.

With a verbal score of 28 you must be having difficulty with more than just the reading comprehension questions. You need to go over the questions you got right as well as the questions you got wrong to understand where you are getting off track. Try analyzing your results to identify concepts that you are consistently missing. Blaming the test will not help you. Understanding your command of the concepts and strategies will guide you toward achieving your best score.

Good luck,