IANAD but my guess is maybe a stronger immune response in the first dose makes the immune system respond so quickly to the second dose, that it doesn't have tim

IANAD but my guess is maybe a stronger immune response in the first dose makes the immune system respond so quickly to the second dose, that it doesn't have time to get anywhere and interact with enough of the body to have much of an effect. That way a weaker immunization in the first dose leaves more room for the second dose to have a refreshing/strengthening effect. Basically, 1 + 2 > 2 + 0