The GMAT is a Computer Adaptive Test (CAT) so there is no way to compare two different tests. Since it is a CAT test, the more problems you get right, the harder the questions you will see. SO if you are seeing harder questions on the real test, than it is very possible you are actually scoring higher.
The fact is though, the GMATprep software pulls from retired GMAT questions, so they are reallly all the same. Everyone says that the GMATprep is the absolute closest thing out there to the real GMAT, so I would consider it very comparable in all areas.
PS. I've never heard anyone say that the GMATprep quant was easier, where did you get this info?