My guess with the Power Prep score is that based on on your performance on the first "medium" quant section, you got the easier second section which does impact your scores, even if you get answers right. I had a pretty similar experience with my practice test scores before I took the GRE on Monday.
Magoosh test- 2 weeks before test Q 149
ETS PP1- 1 week before test: Q 151
ETS PP2- 2 days before test: Q 150 (this is when I started to have a breakdown!)
Magoosh test- 1 day before test: Q 154
Magoosh score estimate: Q 155-160
Actual test: Q 156
In the few days before the test, I watched videos for the two areas I hadn't covered yet-- Coordinate Geometry and Probability and I focused on doing as many problems as possible. When I got problems wrong, I watched Magoosh videos explaining the answers AND re-watched some of the review videos.
The day before the test, I stopped doing work around 4 pm. I wasn't going to learn anything more at that point and I was making myself crazy. I watched a favorite show, ate a good dinner, hung out with my wife. I think that helped.
I do think that the adrenaline on the day of the test helped me focus. In the second quant section, I clicked through a bunch of problems to see what was coming up and started to freak out a little. I took a few deep breaths and reminded myself to (1) make sure I understood what the question was asking and (2) avoid over-complicating things. I have a tendency to lean towards formulas and calculations rather than logic and estimation. There were at least two problems in the second math section that I was able to solve by slowing myself down and using logic.
My goal was 50th percentile in math. I really didn't think I was going to get it. I was shocked when I saw my score. I haven't had a math class in 17 years! If you are anything like me, you can do it!