I have a bunch of thoughts but I am also at an airport about to board, so I will leave them here a bit disorganized. BTW, I interned at FAANG for my summer as a PM and have a fair idea about tech recruiting in top schools.
1. The differences may not be statistically significant. You need to run an actual analysis to see if the differences are of any natural importance.
2. Tech is really weird. People lump different tech jobs into the same tech bracket. For example, tech could mean operations at Amazon, retail at Walmart eCommerce or finance at Microsoft. They are all tech roles but functionally very non-tech, and as a result, have different salaries.
3. Companies do not pay the same for different roles. I will give you one unique example - Amazon PMTs earn about $145K base + $105K sign-on (split in two years) and about $140K (vested in 4 years) which is SIGNIFICANTLY higher than any finance/consulting roles out of business school. Google and Apple also pay similarly - a lower base but a much higher total comp which often does not reflect in the salaries.
4. Schools in west coast will place more for PM positions than other schools, and as a result will draw a higher tech salary.
5. There are industry outliers - Facebook pays a LOT more than any other tech company and does not traditionally hire from MBA (or Ross/Tuck/Stern etc for PM roles).
So all of this taken together, explains some of your disparity. Hope this helps!