The FAQ says that their dataset only goes back until June of 2001, so I imagine the methodology for the prior years in the Very Long Term History section was somewhat different.

The claim at the top of the page is that "The ratings are based on the number of skilled engineers world-wide, courses and third party vendors." However, the details show that it really is just a single search term across a variety of search engines. Maybe they used to have an intern read the classifieds at the back of the Communications of the ACM or something.