Herd Immunity works like nuclear fission. You have some slow rate of spontaneous events (a person getting measles while traveling or a neutron from the environment striking an unstable atom and splitting it), and a chance for each event to create another (that person infects another, a neutron from the first atom strikes another).
For nuclear reactions, the chance depends on how much of the stuff is packed together. The more stuff, the greater the chance that a neutron slams into another atom ready to decay. For disease, the chance depends on how many people the sick person sees and how many of them have been vaccinated.
For both reactions, the critical question is whether the average event creates 0.99 new events or 1.01 new events. After 400 cycles, there is a massive difference between 0.99 ^ 400 and 1.01 ^ 400 (0.02 vs 53.5). A small difference in the percent chance creates a huge difference in the outcome. That is why it matters so much when a chunk of nuclear material goes over the critical mass. If each atom that splits causes 53 more atoms to split, then you'll have a nuclear explosion. If each person who gets sick makes 53 more people sick, then you'll have an epidemic on your hands.
And that's why it matters so much whether 95% of the population has been vaccinated or 90%.