Hi,
and welcome to the board. I am glad to help you
This difference should be an object of investigation. However, did you take the exam in akin test conditions ?? how long did pass from the Manhattan test and the greprepclub test ?? one month ?? one day ??
of course, I am not saying that our test is tougher or better or other silly things because
Manhattan Prep is a well-established brand. Nonetheless, our test is really tailored to mimic very similar to the power prep test.
Another suggestion I could give you is this:
1) do not rely too much on the different mock test because , at the end of the day, they are always un-official tests. The only hallmark you should take for granted is the official test PP
2) try to take both tests in similar stress-conditions: the same hour of the day, silent environment (both; not one with noise or disturbances and the other not) and so forth
We are always here for any possible feedback from you, giving assistance.
Regards