Many vaccinations do not initially provide protection until they are boosted several weeks later.
Protection wanes over time, and eventually, the vaccination will be required again to restore the needed level of immunity.
The number of initial boosters, and their timing, depend upon studies by the manufacturer, under guidelines set by the USDA.
The frequency of revaccination depends on many factors, not the least of which is your specific cat’s immune system. Originally, most vaccinations were recommended to be boosted once annually. We are getting an increasing amount of field data that indicates that for some vaccinations this is not necessary so frequently.
For more details, see the American Association of Feline Practitioners’ vaccination guidelines. This is the basis for the recommendations of many veterinarians. Is it still over-vaccinating? Maybe, but I would rather my cat have a few too many vaccines over 15+ years than to watch him suffer and die from something I could have prevented.