While people will (correctly) focus on the Quant, the verbal is also very different. MGMAT tests allow for significantly more errors than GMAT Prep/Actual Gmat while still giving you harder questions. I believe I got a V45 with around 12 wrong on MGMAT, and this will almost never happen on the real test.
I guess you might be right. They are somewhat different. But I think that mgmat's verbal part is the most accurate at predicting your score. The mode of my verbal scores was 42 and that's exactly what I got on the real exam.
He that is in me > he that is in the world. - source 1 John 4:4