I have two items for you and then a very important question:
1) A reassurance - The Reading Comp is a really difficult question type to create. Unofficial reading comp passages are usually not edited and tested as carefully as the Official Questions. Think about it, but the time a question makes it to the Verbal Review or Official Guide it has been attempted by thousands of people (mostly on the actual GMAT) and edited dozens of times. A single word change can make a question much more difficult, or even make a question unfair. On of the benefits of the new Veritas Question Bank and practice tests is that students are able to leave comments and suggestions on particular questions and have helped us to edit many of questions and passages so that they get better and better over time, just like the Official Questions.
So my reassurance is that the Official questions are a better gauge of your ability.
2) A caution - before we write this all off to a difference between official and unofficial questions, there is another difference you need to think about. The difference between paper and computer!! For many people, the reading comp is easier to do on paper. The text can lay flat, you can trace it with your finger - you might even be marking on it! You cannot do these things with the computer screen. Here is an article I wrote about practicing with paper materials in a realistic way
https://poetsandquants.com/2012/06/28/wh ... -on-ipads/
So how do you know which of these is the problem for you? The question is how do you do on official material that is delivered on the computer screen? How are you on the GMATPrep practice tests? Or the 15 free RC questions on GMAT Prep? Or the GMATPrep Pack 1 add-on that you can purchase to add 400+ total questions (include several more reading comp passages)?
If you are doing well on the official questions on the computer then be reassured. If not, then you need to work on translating your excellent performance on paper over to the computer delivered questions. After all, as I mention in the article that I linked above, until the GMAT is given on the IPad, there is no writing on the computer screen!