Why is the phrase "five score years ago" more appropriate than "a hundred years ago" or in "1863" why does king later repeat the phrase "one hundred years later" so often?

gaius cassius longinus and marcus junius brutus, former enemies of julius caesar who'd joined the roman senate, led caesar's assassination, dubbing themselves "the liberators." and the second one is   the assassination of julius caesar was the result of a conspiracy by many roman senators led by gaius cassius longinus, decimus junius brutus albinus, and marcus junius brutus. they stabbed caesar (23 times) to death in a location adjacent to the theatre of pompey on the ides of march 15 march 44 bc.