This scientific concept was popularized (as a part of Chaos Theory) during the 1980s. In its popular form it's usually quoted as "A butterfly flapping its wings in Brazil may cause a tornado in Texas". The concept was discovered by the scientist Edward N. Lorenz in 1963, during a computer simulation of weather. It was when he entered the almost-precise data from an earlier point in time than that which the calculations had already reached. After continuing to run the simulation for the same amount of time, the results turned out completely different: Proving that small changes don't have to stay small, but may grow exponentially.
(Ray Bradbury's story "A Sound of Thunder" predicted something like this as early as 1952, BTW.)
In Alternate History, many authors like to use well-known historical characters in similar functions as in our world's history. Take Joseph Goebbels as an inquisitor in the service of the Vatican, or Richard Nixon as a used-car salesman - of steamcars to boot. This makes their stories easier to digest, and their characters easier to relate to.
Usage in "How many Sixes..."
Also, it fits - if we use the butterfly effect - that the world we know is destroyed bit by bit as the years go by. (As if the Nazis hadn't done enough damage in the first place.) Hence, the only people who are the same as in our world are those conceived and born before the Point of Divergence. That is: 1940s, roughly. If I decide to let people from our history appear who are born after this date, they'll be really more like siblings from the same parents (possible after all, esp. if these parents were already married/betrothed/an item before the PoD), with the same sex (the chance is about 50%), the same first name (possible, families like to reuse names, esp. traditional ones, and people's tastes tend to stay similar), and roughly the same birth date.
Furthermore, there's still the possibility of "morphic twins", as Terry Pratchett would have said: Different people who have the same function as certain people from our history. If there is no Steve Jobs, someone else will probably still invent the PC.