GMAT Verbal & Essays

I scored consistently between 85-96% in verbal on all of the MGMAT exams (620, 660, 660, 700, 710), but then I took the GmatPrep tests and although my Quant score was a bit higher than MGMAT, my verbal score was around 40%! I went over the GmatPrep exams and could not figure out why my score is soooo much lower than the MGMAT exams. I thought the MGMAT verbal is much harder than GmatPrep?? What am I doing wrong? In the GmatPrep exams the types of questions, especially the sentence correction, look a lot different to me. I need to figure out what Im doing wrong in the verbal section of the GmatPrep exams stat....Please offer your advice...I was also wondering if the algorithm the GMAC uses has anything to do with this....Thanks!!!