I was taught it was to mimic a biblical phrase, with "three score and ten" being the length of someone's allocated life. It was therefore to indicate that America was older than the length of any one person. What do you think Tom?,(reply>), "Three score and ten" is not a Biblical phrase. Psalm 90 does peg the average lifespan at "70 years", but the word it uses for 70 means, literally, "70".