I can explain this, too. The legend in it's entirely is: “Once every 100 years, when the faith in God is forgotten, Dracula will come back to live”. IGA explained once that the "forgetting faith in God" part is what dictates Dracula's ressurection. It just so happens to occur roughly every 100 years. Which means it can be slightly off by a few years.
That could also explain Dracula's resurrections in the 19th-20th centuries becoming more frequent.