Why are the dates in Les's Bible timeline approximate?
The timeline dates are approximate because they are not set in concrete; across thousands of years, a variance of about a hundred years either way makes little difference. They represent the closest estimates chronologists can ascertain, such as Adam appearing around 4004.
Les explains that he puts the timeline on the board starting in Genesis 1, where Adam was probably created. He notes all these dates are approximate and not set in concrete, because over thousands of years a hundred-year variance one way or the other doesn't make much difference. As near as chronologists can ascertain, Adam came on the scene at 4004, and the Flood, the next big event in Genesis chapters 6 and 7, was about 2,400 years later.