when is the webpage number on google homepage changed?

is it changed when google is updated?

[Noximus]

dear friends i mean the text below
"©2002 Google - Searching 2,469,940,685 web pages"
is it changed when google is updated? is it exact sign of google update?

[Noximus]

Dear Experts,
please tell me the answer if you know :)

[martin]

They change it whenever they have some spare time to count all pages ;-)

[Noximus]

thank you Martin :)

[ciml]

I guess Martin's pretty close to the truth. I wonder why they don't change it more often, maybe they don't want all the 'has it updated' emails?