when the serps change you most of the times get a PR and back links update - means how many other sites link to your site and its just one of the many ways SE rank your site and they only give you so many per update and only let you see so many to get a more accurate no. go to yahoo and type in the link back command link:
http://www.yoururl.com/ and having a lot helps.. and the way you get them..
but there are sites that have a low no. and rank well in the serps and vice versa and its just a small piece of the puzzle to have you site rank decent in the serps..