How does 7Sage calculate the PrepTest equivalent score?

I got -4 on PT138.S4, and my "preptest equivalent" score was a 164. Then I took PT104.S1, and I also got -4, but the "preptest equivalent" score is 169. This seems like a big jump for the same number of questions. Does this just indicate that PT104.S1 was a harder section than most?

Does anyone have any tips for Strengthen/Weaken questions on Phenomenon Hypothesis passages? I do not struggle with the concept of strengthening or weakening on most questions, but I seemed to get tripped up on harder WSE questions in a phenomenon hypothesis passage.