Why does the 18th century refer to the 1700's? This is one thing in history I just don't understand. Would the 1700's be the 17th century?

Becouse first century AD had no 'hundred' in numeral so let sie Nero seting fire in Rome and persecution of Christians took place in year 64 in 1st century there is no 'hundred number' nad for exempla emperor Traian died in 117 in second century, as You see in first century there is number in place where hundreds shoudl be, it's simple math