[Symmetry]

After exposure to a pathogen the levels your body starts evolving antibodies against it. It also quickly ramps up antibody production, but ramps that down over time without additional exposure.

Having had a more recent jab will mean a higher quantities of antibodies. But more vaccinations also means your body has done more cycles of evolving better targeted antibodies against the threat. I assume that there's a certain level of diminishing return to the second factor, as well as questions about how much the extra work translates against a new variant. But the second effect exists alongside the first.