And it turns out the cohort who received the half dose was on average younger than the total study population, this explaining the apparent improvement in efficacy
As far as I can tell, the virus is more prevalent among younger people. Since infection, not death, was the primary endpoint, and also because medicine hasn't used that sort of wishful reasoning for about a hundred years, the age difference doesn't explain anything.
it certainly does, younger people have stronger immune systems, and therefore generally react better to vaccines than older people. The stronger the immune system reacts to a vaccine the better, since that usually ensures that there will be an immune-response to the actual virus as well.