How does a judge come up with 431 years?

I just read that Phillip Garrido (the Jaycee Dugard kidnapper) got sentenced to 431 years in prison. How did the judge come up with that number? I also heard the Jeffry Dahmer got 957 years....Is there some complex math involved in calculating exactly how long he or his zombie will be in jail? Or did the judge just come up with some huge sounding number?