I consider myself pretty knowledgeable about Disney tributes and trivia... but this one has always perplexed me as well. I think it's based on how many people had been in the MK since it opened... but at one point they decided to stop updating it.
disneylands looks like it says population 8,000,000. Paris doesn't have a population on it. I was looking for old photos and it doesn't appear to have changed over the years although the older the photo the harder it is to read but seems like the same number of digits.
@Merry Mousketeer do you have any insight into the population numbers on the train station signs?
Ok I found something.. Apparently the sign used to say 600,000,000 and they did a big celebration around the 600,000,000 guest. My guess is every 100 millionth guest they change it.
source: http://www.answers.com/Q/Why_does_the_Magic_Kingdom_population_600000000?#slide=1 (scroll the slides, it's a horrible way to see the answer but it also contains some guest numbers over the years..) It even guestimates that as of 2012 it would have passed 800,000,000 so when was the last time the station was freshened up?