It is possible to have a lower average difficulty for wrong answers but you don't have this problem. There's no statistical difference between 680 and 670. Essentially, that's the same level of difficulty.
That's not unusual - we all have strengths and weaknesses in different areas. It is more ideal, yes, to have a higher difficulty for wrong answers. You likely either have some holes in your foundation that are pulling down your performance a bit or you are making careless mistakes on easier questions that you do actually know how to do (and then your average difficulty for wrong answers goes down).
You'll have to go through the individual problems to figure that out. Where are you making careless mistakes and why? Figure out how to minimize them in future:
http://www.manhattangmat.com/blog/index ... -the-gmat/You'll also need to look at your weakest areas and try to build them up. You can use this to analyze your practice CAT:
http://www.manhattangmat.com/blog/index ... ice-tests/If you'd like, you can then come back here and tell us the results of your analysis and what you think you should do based on that analysis. We'll tell you whether we agree and advise you further. (Note: do share an analysis with us,
not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)