Yes, reserve is a big unit if it is actually available. When I removed my stock tank to install the XT350 tank, I drained as much fuel as I could from the tank (disconnected the fuel line and opened the petcock). Then I took the tank off and attempted to drain the rest by placing the tank on a flat surface and opening s the petcock again. When I really got violent and started tipping the tank and trying for all of the liquid, I got a significant amount more (sorry its been years), but the point is that even with reserve exhausted, there is more gas in the tank and tipping it will get you more gas.
Do I believe your 120 mpg? Yes but to publish it in the real world you would need details, circumstances, and ways to verify. Do I sound like a former research scientist?
100 mpg is easy with careful riding, and a well tuned bike, and 120 might be possible but tell us about it. Congrats in the meantime. Tom