The TIOBE index is based on the number of search results for a programming language across popular search engines, which is pretty limited.
Basically the calculation comes down to counting hits for the search query:
Python has won the prestigious TIOBE Programming Language of the Year award. Congratulations! This is the second time in a row. The award is given to the programming language that has gained the highest increase in ratings in one year. C# was on its way to get the title for the first time in history, but Python surpassed C# in the last month.
Read the full article on the official website.