With respect to the TIOBE index, they use a different methodology drawing from a large number of search engines. As an example of how this can make a difference, if you look at TIOBE's historical graph you will see a huge shift back in 2004 when they made a major change in how they determined the rankings. You can see that over time they are not necessarily even consistent with themselves.
Redmonk on the other hand use a much simpler method with only two data sources. However, their methodology does try to distinguish between languages that are talked about versus languages that people are actively writing code in.
TIOBE sell access to their raw data set, so they have an incentive to make it as big and complex as possible.
Neither method is really "correct" in the sense of counting how much code is being written in which language, but they do provide a general idea of which languages are all hype versus which ones are seeing actual use, and they give an idea of overall trends.
On either list, once you get out of the top 10 you are looking at languages which are not seeing widespread current use outside of niche applications or else are on their way out.
Biting the hand that feeds IT
