If you look at some of their published Phase 2 data, they were actually getting good responses from their 25mcg dosing. But you're correct--this was the fastest vaccine development in the history of mankind, and they errored on doing the 100mcg version just to make sure the efficacy was high enough because they didn't have time to test it on a wider population or do challenge trials.

Imagine the blowback on mRNA vaccines in general if they went with the low-dose version and got lowered efficacy...