an important thing to know about the ancient romans is that they couldn't fucking count

one could argue that they simply had a different convention by which they counted intervals inclusively (e.g. their 8 day cycle of market days being called "nundinae").

in counterpoint i offer that for the first thirty-six years after Julius Caesar reformed their calendar to have a regular leap day every four years, the priests in charge of the calendar accidentally did it every three years instead.