This is true for ALL immunizations requiring boosters, in both man and beast. The initial immunization (shot) causes antigens to whatever it is to be developed in the body. With some, that protection can fade over time, and a "booster" is required to bring the protection up to the full strength again.
There's no difference. They just need a booster every now and then, because the body can "forget" the immunity it learned from the first shot. Or, in the case of kittens, they need multiple shots because the first shots may not take because they still have immunity from their mother.
Also, cats can't get distemper. We tend to refer to panleukopenia as "feline distemper" but it isn't related to distemper (it's a parvovirus). Most vets understand that and know what you mean by "distemper vaccine" but if someone looks confused, the combo vaccine that's most commonly used is FVCRP.