It's sort of unusual that you would find Power Prep to be harder (score lower) than Kaplan's CD-ROM. Most people find that they score much, much lower on Kaplan than they do on PP or the real GMAT. [I'm not sure if there is a consensus regarding other companies's software.] I'm not quite sure what to think or to advise.
As for the practice questions on PP vs the PP CATs, the practice questions are A LOT easier than the CAT questions on PP or the real GMAT. I'm not sure what GMAC was thinking in making the practice questions SO easy relative to the practice tests and the real one. I guess they just have a limited supply of questions they are willing to release at any one time so a lot of problems that don't fit into the practice CATs wound up in the practice questions section.